Every entry below is the rule as Ireland publishes it. It is a reference, not a calendar: where a deadline runs from something only you know, this page says so instead of showing a date.
This is the rule as published by the registry, checked against the primary source on the date shown. It is not your filing date. Where a deadline runs from incorporation, from a fiscal year the company itself elects, or from an event only you know about, no date can be derived here at all — the figure you see is the formula, not the answer. Extensions, transitional regimes and entity-specific exemptions are not reflected. Confirm with the registry before you rely on a date.
An Irish company sets its own financial year. The registrar states the rule as a first period running from incorporation and ending no later than 18 months after it, with each subsequent financial year beginning immediately after the previous year end and running no more than 7 days shorter or longer than 12 months. The year end can be moved by filing Form B83 (fee 15 euro), which the registrar will not accept if the resulting financial year would exceed 18 months, if the delivery deadline for that year's financial statements has passed, if it would leave a gap in the periods covered, if it would mean an annual return is never filed, or if less than 5 years have passed since a previous B83 (with an exemption for an EEA subsidiary or holding undertaking aligning its year). The financial year is therefore a date the user must supply, and no deadline counted from it is shown here as a calendar number. Two separate clocks run in Ireland and must not be confused: the annual return runs from the company's Annual Return Date (ARD), which is derived from the date of incorporation and not from the financial year end, while the corporation tax return runs from the end of the accounting period. The financial statements are what ties them together — the registrar requires that documents annexed to an annual return be «made up to a date falling not more than 9 months before the date to which the return is made up».