Form CT1: Corporation Tax Return
Annual Corporation Tax return for Irish companies, filed via ROS within 9 months of accounting period end (specifically the 23rd of the ninth month for ROS filers). The return is due nine months after the accounting period ends, and the balance of tax is due on the same day. Companies must file and pay through ROS under mandatory e-filing, so the 21st day applies only to the rare filer who is not on ROS.
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Who this applies to
- All Irish-resident companies
- Non-resident companies trading in Ireland through a branch
- Investment vehicles within scope of CT
What to file
Form CT1 with full computation, Statement of Profit and Loss, Statement of Financial Position, schedules for capital allowances, R&D credit, losses, and dividends paid. Revenue lists Form CT1 and Form 46G (Company) as the forms to file by the return date.
How to file
Online through Revenue Online Service (ROS) at ros.ie. iXBRL accounts required for medium and large companies, optional for small companies.
Payment due
Balance of CT due same date as return. Preliminary tax (90% current year or 100% prior year for small companies) was due one month before year-end.
Penalties for missing this deadline
5% surcharge of CT liability (capped €12,695) for late filing within 2 months. Increases to 10% (capped €63,485) if more than 2 months late. Plus interest on unpaid tax at approximately 8% per annum. A late return also restricts claims for excess capital allowances, loss relief and group relief, with the restriction growing with the delay. Interest runs at 0.0219% a day on unpaid corporation tax, cannot be appealed to the Tax Appeals Commission and cannot be reduced once charged.
Filing checklist
- Sign in to ROS with your digital cert
- Open the Form CT1 for the relevant accounting period
- Complete trading profits, capital allowances and credits
- Upload iXBRL accounts (mandatory if not a 'small' company)
- Submit Form CT1 and pay any balance via ROS
- Keep filing receipt: Revenue retain electronic record
- File the iXBRL accounts before the CT1, with it, or within 3 months after the CT1 due date. Late iXBRL accounts can make the CT1 late
Documents you'll need
- Trial balance and adjusted profit calculation
- Capital allowances schedule
- iXBRL-tagged financial statements (medium/large)
- R&D tax credit claim if applicable (Form RDT)
- Group relief notices if applicable
- Form 46G (Company) details, filed with the CT1
Common mistakes to avoid
- Missing the 23rd of the month deadline (the 21st applies for paper filers)
- Forgetting preliminary tax was due 1 month before year-end
- Filing without iXBRL when required (auto-rejected)
- Not claiming all available credits (R&D, KEEP, FED)
- Submitting draft financial statements in iXBRL format: Revenue will no longer accept drafts from 1 January 2026
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