Corporation Tax (CT1)
aka CT1, Irish Corporation Tax return
The annual Corporation Tax return filed by Irish resident companies through ROS. Trading income is taxed at 12.5% (or 15% for large groups in scope of Pillar Two); passive income and certain non-trading profits are taxed at 25%.
Last reviewed April 2026
Definition
The CT1 is the annual Corporation Tax return that every Irish resident company must file with Revenue through ROS, within nine months of the end of its accounting period (and in any case by the 23rd of the ninth month for ROS filers). Trading income is taxed at 12.5%; passive income, Irish rental income and certain non-trading profits are taxed at 25%. From 1 January 2024, large multinational groups with consolidated revenue of at least EUR 750 million are within scope of the OECD Pillar Two global minimum effective tax rate of 15%, introduced in Irish law via the Finance (No. 2) Act 2023. Preliminary Corporation Tax is due by the 23rd of the eleventh month of the accounting period: small companies (prior-year CT under EUR 200,000) can base it on 100% of the prior year; larger companies must pay 90% of the current year estimate in two instalments. The CT1 reports taxable profits, claims reliefs such as R&D tax credit and the Knowledge Development Box, and reconciles to the statutory financial statements.
Why it matters for software choice
For most Irish SMEs the CT1 is filed by an external accountant, but the quality of the underlying accounting records directly determines the cost and time of preparing it. Software that produces clean trial balances, correctly segregates trading vs non-trading income, and tracks R&D eligible costs year-round gives the accountant everything they need in one export and removes the end-of-year scramble.
Authority sources
- Revenue: Corporation Tax (www.revenue.ie)
- Revenue: Pillar Two global minimum tax (www.revenue.ie)
Software categories this affects
Vendors covered by this term
Xero
Cloud accounting with direct AIB, BOI, and PTSB bank feeds for Irish SMEs
Surf Accounts
100% Irish-built cloud accounting with Revenue compliance and Dublin-based support
Sage
Ireland's most established accounting platform with built-in RCT and PAYE support
Related terms
ROS (Revenue Online Service)
Revenue's secure portal for businesses, agents and large filers. Used to file VAT3, Form 11, CT1, RCT notifications, PAYE submissions and to access Revenue Payroll Notifications (RPNs).
Income Tax Return (Form 11)
The annual self-assessment income tax return for self-employed individuals, company directors with material interests, and taxpayers with non-PAYE income. Filed through ROS by 31 October (or mid-November via ROS Pay and File extension).