Quick answer: Revenue requires companies to file a Corporation Tax return using Form CT1. The return and any balance of tax due are generally due nine months after the end of the accounting period, with the ROS deadline falling on or before the 23rd of the ninth month where that rule applies.
Reviewed: 14 October 2026. This guide is general information, not tax advice.
Reviewed by Saiful Alam Sabuj M.
ACCA-qualified · Managing Partner and Accountant at Stratancy Consulting
What is a CT1 return?
Form CT1 is the Corporation Tax return used by Irish companies to report the company's taxable results for an accounting period. Revenue requires companies subject to mandatory e-filing to submit the return through Revenue Online Service (ROS).
When is a Corporation Tax return due?
Revenue states that a company must file its return and pay any balance of Corporation Tax due nine months after the end of the accounting period. For ROS filers, where the due date falls under the month-end rule, the deadline is on or before the 23rd day of the ninth month.
The exact date should be checked against the company's accounting period end. The CT1 deadline is separate from the company's CRO Annual Return Date and B1 filing obligation.
What is preliminary Corporation Tax?
Preliminary Corporation Tax is generally paid before the final CT1 filing deadline. Revenue applies different timing rules to small and large companies. A small company, for this purpose, is one whose previous accounting period Corporation Tax liability is less than €200,000.
Revenue states that a small company normally pays preliminary tax in one instalment 31 days before the end of its accounting period and before the 23rd day of that month.
What records are needed for the CT1?
- Complete bookkeeping records for the accounting period.
- Year-end bank and balance sheet reconciliations.
- Payroll, VAT and other tax records where relevant.
- Details of fixed assets, finance costs and material expenses.
- Information about directors, related parties and exceptional transactions where relevant.
- Statutory financial statements and supporting tax computations.
CT1 and B1 are different filings
The CT1 is filed with Revenue for Corporation Tax purposes. The B1 Annual Return is filed with the Companies Registration Office. Both can form part of the same annual compliance cycle, but they have different rules and deadlines.
Official Revenue sources
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