<PAGE>

                                                                    EXHIBIT 12.1


                                  CERTIFICATION

I, Christopher J. Horn, certify that:

      1. I have reviewed this annual report on Form 20-F of IONA Technologies
PLC;

      2. Based on my knowledge, this annual 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 annual
report;

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

      4. The registrant'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 the
registrant 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 the registrant, including
      its consolidated subsidiaries, is made known to us by others within those
      entities, particularly during the period in which this annual 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 the registrant'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 annual report based on such evaluation; and

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

      5. The registrant's other certifying officer and I have disclosed, based
on our most recent evaluation of internal control over financial reporting, to
the registrant's auditors and the audit committee of the registrant'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 the registrant'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 the registrant's internal
      control over financial reporting.


By: /s/ Christopher J. Horn
   ------------------------
   CHRISTOPHER J. HORN
   CHIEF EXECUTIVE OFFICER

Date: May 14, 2004