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|Exhibit 12.1

Certification

Pursuant to Section 302 of the Sarbanes-Oxley Act of 2002

I, David Duffy, Chief Executive Officer, certify that:

1. I have reviewed this
Annual Report on Form 20-F of Allied Irish Banks, p.l.c. (AIB);

2. Based on my knowledge, this report does not contain any untrue statement of a
material fact or omit to state a material fact necessary to make the statements made, in light of the circumstances under which such statements were made, not misleading with respect to the period covered by this report;

3. Based on my knowledge, the financial statements, and other financial information included in this report, fairly present in all material respects the financial
condition, results of operations and cash flows of AIB as of, and for, the periods presented in this report;

4. AIBs other certifying officer and I are
responsible for establishing and maintaining disclosure controls and procedures (as defined in Exchange Act Rules 13a-15(e) and 15d-15(e)) and internal control over financial reporting (as defined in Exchange Act Rules 13a-15(f) and 15d-15(f)) for
AIB and have:

(a) Designed such disclosure controls and procedures, or caused such disclosure controls and procedures to be designed under our
supervision, to ensure that material information relating to AIB, including its consolidated subsidiaries, is made known to us by others within those entities, particularly during the period in which this report is being prepared;

(b) Designed such internal control over financial reporting, or caused such internal control over financial reporting to be designed under our
supervision, to provide reasonable assurance regarding the reliability of financial reporting and the preparation of financial statements for external purposes in accordance with generally accepted accounting principles;

(c) Evaluated the effectiveness of AIBs disclosure controls and procedures and presented in this report our conclusions about the effectiveness of
the disclosure controls and procedures, as of the end of the period covered by this report based on such evaluation; and

(d) Disclosed in this
report any change in AIBs internal control over financial reporting that occurred during the period covered by this report that has materially affected, or is reasonably likely to materially affect, AIBs internal control over financial
reporting; and

5. AIBs other certifying officer and I have disclosed, based on our most recent evaluation of internal control over financial reporting, to
AIBs auditors and the audit committee of AIBs board of directors (or persons performing the equivalent functions):

(a) All significant
deficiencies and material weaknesses in the design or operation of internal control over financial reporting which are reasonably likely to adversely affect AIBs ability to record, process, summarize and report financial information; and

(b) Any fraud, whether or not material, that involves management or other employees who have a significant role in AIBs internal control over
financial reporting.



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|Dated: 25 April 2014 |  |  |  | DAVID DUFFY                   
|                     |  |  |  | Name: David Duffy             
|                     |  |  |  | Title: Chief Executive Officer