EXHIBIT 10.1

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Michael Malecek

                                May 31, 2019

Dear Mike:

I am pleased to confirm this offer for you to join Prothena Biosciences Inc
(“Prothena” or the “Company”). We are confident in your knowledge, expertise and
judgment, and believe your performance will meet our team’s high-quality
objectives and standards.

You will be appointed as Chief Legal Officer and Company Secretary of Prothena
Corporation plc by the Board of Directors of Prothena Corporation plc at its
next meeting, scheduled to be held in early July. In this position, you will
report to Gene Kinney (President and CEO), although your duties, title and
reporting relationship may change, based on the Company’s needs and priorities.
This is a full-time, exempt position - which means that you are not eligible for
overtime pay under state and federal laws.

Your starting annualized salary will be $400,000.00 (gross), paid twice per
month. Your pay is subject to applicable taxes and withholdings.

We agree to pay you a Retention Bonus of $125,000.00 (gross) which will be fully
earned in the event that you remain employed with Prothena for one year. The
entire Retention Bonus will be paid to you with your first paycheck, less
applicable taxes and withholdings. This Retention Bonus will be earned on a
pro-rata basis for each week worked during your initial one-year period of your
employment at Prothena. The unearned portion of this Retention Bonus will be
repayable by you to Prothena on a pro-rata basis if you cease employment at
Prothena prior to the one-year anniversary of your employment start date, unless
your employment ends for a reason that would make you eligible for severance
benefits in accordance with the Prothena Biosciences Inc Amended and Restated
Severance Plan. The forgoing will be set forth in a Retention Bonus Repayment
Agreement, a form of which is attached (Exhibit 1), that we will need you to
sign on your start date in order to receive the Retention Bonus.

Prothena embraces a pay-for-performance philosophy. All employees are currently
eligible for an annual cash bonus under the terms of the Company's cash
incentive plan (the Prothena Corporation plc Amended and Restated Incentive
Compensation Plan). The amount of these annual cash bonuses is determined by the
Company on the basis of a number of factors, including industry competitiveness,
Prothena’s business strategy, and the degree to which Company, function and/or
individual goals are met. Your targeted cash bonus for our 2019 performance year
will be $160,000.00 (gross), which is 40% of $400,000.00. A condition of earning
any cash incentive award is that you remain employed through the pay date of an
otherwise earned award, which will be paid no later than March 15, 2020. The
cash bonus plan is operated at the sole discretion of Prothena, is subject to
review on a regular basis and may change from time to time.

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EXHIBIT 10.1

In connection with your commencement of employment, you will also be eligible to
receive an option to acquire 250,000 shares of Prothena Corporation plc. This
stock option award is at the discretion of the Compensation Committee of the
Board of Directors of Prothena Corporation plc (the “Committee”) and is subject
to the terms and conditions of the Prothena Corporation plc 2018 Long Term
Incentive Plan and the terms and conditions of the award agreement for such a
stock option. The grant date of this stock option will be the date you are
appointed Chief Legal Officer and Company Secretary of Prothena Corporation plc.
The option exercise price will be equal to the closing price of Prothena
Corporation plc’s ordinary shares on the NASDAQ Global Select Market on that
date. Subject to your continued employment, the stock option will vest 25% on
the first anniversary of the grant date, and monthly at a rate of 1/48th of the
award thereafter, such that the option will fully vest after a four-year period
following the grant date.

On the first day of the month following your employment start date, you will be
eligible to participate in Prothena’s comprehensive health and welfare benefits
program. On your start date, you will also be eligible to participate in our
retirement benefits plan, as well as the Prothena Biosciences Inc Amended and
Restated Severance Plan. Details about these and other applicable plans will be
provided separately.

The Company provides paid vacation time to full-time employees in accordance
with the Company’s vacation policy in its Employee Handbook, which will be
provided to you upon commencement of your employment. You will also be eligible
for paid sick time as required by state law. Additional information about paid
sick time is contained in the Company’s Employee Handbook. Among the policies
applicable to the Company’s employees is an Arbitration Procedure, which
requires that disputes be submitted to final and binding arbitration for
resolution. A copy of the Arbitration Procedure is attached (Exhibit 2).

Further information regarding onboarding requirements and/or documents needed on
your employment start date (e.g., Employee Proprietary Information and Invention
Assignment Agreement, Code of Conduct, Form I-9 completion process, direct
deposit information, Form W-4 allowance elections) will be provided separately.

This offer is contingent upon your successful completion of a background check
and a pre-employment drug test. More information regarding this process will be
provided by Human Resources.

Additionally, your acceptance of this offer of employment and commencement of
that employment means that you understand and agree that your employment
relationship with the Company is at-will, for no specific period, and neither
this letter nor any other oral or written representations may be considered a
contract of employment for any specific period of time. As a result, you are
free to resign your employment with Prothena at any time, for any reason or no
reason. Similarly, Prothena is also free to end your employment at any time,
with or without cause or advance notice. At-will employment also means that the
Company may make decisions regarding other terms of your employment at any time
with or without advance notice or cause, including but not limited to demotion,
promotion, transfer, discipline, compensation and duties. Further, all benefits
and compensation provided by the Company are contingent upon your continued
employment.

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EXHIBIT 10.1

To accept our offer, including agreement to the Arbitration Procedure, please
sign this letter and return it to Kevin Hickey (VP, Human Resources) by June 7,
2019. This offer is valid until then, after which time we will not be able to
accommodate an acceptance of this offer. Accordingly, please sign and return
this letter before the above-stated expiration date. If you do not intend to
accept this offer, we would like to be notified as soon as possible.

This letter, along with the Company’s policies and procedures, including the
Arbitration Procedure, sets forth the terms of your employment with the Company
if you accept this offer and commence that employment, and supersedes any prior
representations or agreements, whether written or oral. This letter may be
modified only by a written agreement signed by you and an authorized officer of
the Company.

We look forward to having you join Prothena as a full-time employee. If you have
any questions, or if you would like additional information to help you reach a
decision, please feel free to contact Kevin at (650) 837-8545. Please be sure to
bring with you on you first day of employment documentation that proves your
eligibility to work in the U.S., your bank details and emergency contact
information.

Sincerely,

/s/ Gene Kinney

Gene Kinney
President and CEO
Prothena Biosciences Inc

ACCEPTANCE:

/s/ Michael Malecek              June 4, 2019
Michael Malecek                    Date

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EXHIBIT 10.1

Exhibit 1 - RETENTION BONUS REPAYMENT AGREEMENT

I, Michael Malecek, will receive $125,000.00 (gross) as a Retention Bonus with
my first paycheck, less applicable withholdings and deductions, from Prothena
Biosciences Inc (“Prothena” or the “Company”), which will not be fully earned
until I have provided 12 months of Active Service as described below.

I understand and agree that a key purpose of the Retention Bonus is my retention
as an employee, that the Retention Bonus is being paid to me before it is
earned, and that I have not earned the Retention Bonus until I complete 12
months of continuous Active Service. For purposes of this Retention Bonus,
“Active Service” includes periods from my employment start date that I am
continuously employed by the Company or its affiliates, including time off for
approved vacation, holidays, personal time, family & medical leave, and military
leave, but not other leaves of absence unless otherwise required by applicable
law.

I understand and agree that if I complete 12 months of Active Service from my
employment start date, then I have earned this Retention Bonus and have no
obligation to repay any portion of it upon termination of my employment.

I further understand and agree that if my employment terminates for any reason
(other than if my employment ends for a reason that would make me eligible for
severance benefits in accordance with the Prothena Biosciences Inc Amended and
Restated Severance Plan) prior to my completion of 12 months of Active Service
from my start date, then I must repay a pro-rata amount of the Retention Bonus
in one lump sum within 30 days of my termination date. Within 5 days of my
employment end date, the Company will notify me in writing of the amount to be
repaid.

If I fail to repay the amount due under this Retention Bonus Repayment Agreement
(this “Agreement”) within 30 days of the termination of my employment, then the
Company may bring legal proceedings against me for collection. I further agree
that for any claims brought by the Company to enforce the terms of this
Agreement, the prevailing party will be entitled to costs and reasonable
attorneys’ fees.

I agree and understand that nothing in this Agreement alters the at-will nature
of my employment with Prothena, meaning that my employment is for no definite
period and may be terminated either by me or the Company at any time, with or
without cause or advance notice.

I acknowledge that I understand the terms of this Agreement, that I have had an
opportunity to consult with counsel or another advisor prior to signing it, and
that I agree to abide by its terms. I am voluntarily signing this Agreement.

ACCEPTANCE:

/s/ Michael Malecek              June 4, 2019
Michael Malecek                    Date

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EXHIBIT 10.1

Exhibit 2 - ARBITRATION PROCEDURE

Any and all claims, disputes or controversies between you and the Company
arising out of or relating to your employment with the Company and/or the
termination of such employment (“covered claims”), whether brought by you or the
Company, shall be resolved, to the fullest extent permitted by law, by final,
binding and confidential arbitration in San Francisco, CA conducted before a
single arbitrator by JAMS, Inc. (“JAMS”) or its successor, under the then
existing JAMS Employment Arbitration Rules & Procedures.
https://www.jamsadr.com/rules-employment-arbitration/english.

Covered claims include those that you may have against the Company and those
that the Company may have against you. Covered claims include, but are not
limited to claims of discrimination, harassment, and compensation. Covered
claims, however, do not include claims for unemployment insurance, workers’
compensation claims, claims under the National Labor Relations Act, or any other
claim that is not arbitrable under applicable law. As used in this Arbitration
Procedure, the term “Company” includes its subsidiaries, parents, related
entities, officers, directors, employees and agents. A covered claim must be
filed with the party against whom the claim is brought and JAMS within the
applicable limitations period.

Arbitration pursuant to this Arbitration Procedure shall be the exclusive remedy
for resolving covered claims, and the parties mutually waive their right to a
trial before a judge or jury in federal or state court in favor of arbitration
under this Arbitration Procedure. However, either you or the Company may seek
injunctive relief in court to prevent irreparable harm pending the conclusion of
an arbitration.

The rights of the parties under this Arbitration Procedure shall be the same as
those available to them in a court of competent jurisdiction. The decision of
the arbitrator shall be final and binding on all parties.

As the Company is engaged in interstate commerce, this Arbitration Procedure and
the rights of the parties shall - except as specifically provided for herein -
be governed by the Federal Arbitration Act and the California Arbitration Act,
or a combination of the two. The enforcement of the provisions of the
Arbitration Procedure, including those that waive the ability to proceed on a
class, collective or representative basis, shall be interpreted and applied by a
court of competent jurisdiction, and not by an arbitrator.

The court’s jurisdiction may be invoked by the filing of a petition or motion to
compel arbitration. The decision or award of the arbitrator may be confirmed,
vacated, or modified by a court of competent jurisdiction pursuant to the
Federal Arbitration Act, the California Arbitration Act, or a combination of the
two. In the event that the Federal Arbitration Act and the California
Arbitration Act conflict, the one that favors arbitration to the greatest extent
shall apply.

It is the intent of the parties that any dispute covered by this Arbitration
Procedure will be arbitrated on an individual basis, and, unless prohibited by
applicable law, the parties mutually waive their right to bring, maintain,
participate in, or receive money from, any class, collective, or representative
proceeding. Further, no dispute between an employee and the Company may be
brought in arbitration under this Arbitration Procedure on behalf of other
employees as a class or collective action or other representative proceeding.
The arbitrator may not preside over any form of a class, collective, or
representative proceeding.

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EXHIBIT 10.1

With the written consent of all parties, the arbitrator may consolidate claims
filed by multiple individual employees, each on the employee’s own behalf, in a
single arbitration proceeding, so long as the arbitrator does not certify
(conditionally or otherwise) a collective, class, or representative action that
includes individuals who have not themselves already submitted their own
individual claims. In the event that an employee brings both a non-arbitrable
Private Attorneys General Act (“PAGA”) claim and a related wage-hour claim
covered by this Arbitration Procedure, the PAGA claim shall be stayed until the
conclusion of the related arbitration.

In the event the foregoing waiver to proceed in arbitration on a class,
collective, or representative basis is found to be unenforceable or contrary to
law, then any claim brought on such a basis must proceed in a court of competent
jurisdiction, and the court, not an arbitrator, shall be the exclusive forum for
any class, collective, or representative claim.

You and the Company may be represented by legal counsel at any arbitration
proceeding.

The arbitrator shall:
i.
have the authority to compel adequate discovery for the resolution of the
dispute and to award such relief as would otherwise be available under
applicable law in a court proceeding;

ii.
hear and decide all motions, including motions for summary judgment and summary
adjudication of issues; and

iii.
issue a written statement signed by the arbitrator regarding the disposition of
each covered claim and the relief, if any, awarded as to each covered claim, the
reasons for the award, and the arbitrator’s essential findings and conclusions
on which the award is based.

The parties will share the costs of arbitration equally except that, to the
extent required by law, the Company will bear the cost of the arbitrator’s fee
and any other type of expense or cost that the employee would not be required to
bear if the employee were to bring the dispute or claim in court. Both parties
will be responsible for their own attorney’s fees, and the arbitrator may not
award attorney’s fees unless a statute or contract at issue specifically
authorizes such an award.

In the event that any paragraph or provision within a paragraph of this
Arbitration Procedure is determined to be illegal or unenforceable, such
determination shall not affect the validity or enforceability of the remaining
paragraphs or provisions within a paragraph, all of which shall remain in full
force and effect. If necessary, to effectuate the intent of the parties to
resolve the specified disputes through arbitration, a court of competent
jurisdiction should reform this Arbitration Procedure.