Ireland CT1 Countdown: 23 September 2026 for December Year-Ends

For companies whose accounting period ended on 31 December 2025, Revenue’s corporation tax clock now points at 23 September 2026. Revenue’s payment and filing guidance is clear: under mandatory e-filing you must file the return and pay any tax due nine months after the accounting period ends, on or before the 23rd of that ninth month, through the Revenue Online Service (ROS).

That package is more than a tax computation. A complete CT1 filing typically includes Form 46G (Company) and, where required, iXBRL-tagged financial statements. Errors in tagging can reject a return even when the tax figures are correct — leave validation time before the 23rd.

Revenue requires companies to calculate and pay preliminary tax by the earlier specified due date, then complete and file Form CT1 and Form 46G by the return filing date, and pay any balance of tax due on that same filing date. December year-end companies should already have settled preliminary tax; September is about the return, attachments, and true-up.

Interest runs at a daily rate of 0.0219% on late or incomplete payments. Late returns attract a surcharge of 5% of the tax due (capped at €12,695) if filed within two months of the deadline, or 10% (capped at €63,485) if later. Late filing can also restrict claims to excess capital allowances, loss relief, and group relief — so diary the ROS upload, not only the bank payment.

Lock management accounts and book-to-tax adjustments now. Agree add-backs, capital allowances, R&D credit elections where relevant, and close-company issues before the iXBRL pack is frozen.

Confirm your ROS digital certificate is current and that the person who will sign the CT1 can access the company ROS inbox. Open any missing CT periods through MyEnquiries early — Revenue notes that payments applied to the wrong period can be refunded rather than reallocated automatically.

Gather Form 46G contractor and third-party payment details alongside the CT1. Treat 46G as part of the same project plan, not a last-day surprise.

If your group is in scope for Pillar Two top-up tax, keep GloBE data collection on a parallel track — CT1 deadlines and Pillar Two information returns are related but not identical obligations.

Clean books shorten the CT1 review cycle. FinnAccountings helps Ireland and UK limited companies and their advisers prepare bookkeeping and tax packs with Chartered Accountant insight before qualified sign-off. Start a free trial to organise transactions and draft review packs — we prepare records; we do not file your CT1 through ROS or sign off corporation tax returns on your behalf.

Start a free trial →

If you claim the R&D corporation tax credit at 35% for later accounting periods, keep credit elections aligned with the CT1 pack. For VAT and payroll mid-month dates that still fall before September, use the mid-August ROS checklist.

Ireland R&D corporation tax credit at 35% →

Mid-August PAYE and VAT 3 dates still sit ahead of the CT1 deadline — keep ROS payroll and VAT work moving in parallel.

Ireland mid-August ROS PAYE and VAT deadlines →

Use the dual-market tax calculator for high-level planning estimates only — not a CT1 computation.

Open free tax calculator (Ireland & UK) →