Thirty Days to 23 September: Ireland CT1 for December Year-Ends
As of 24 August 2026, Irish companies with a 31 December 2025 accounting period have thirty days to file Form CT1 and pay any corporation tax balance through ROS by 23 September — lock iXBRL and Form 46G after yesterday’s ROS cash cluster.
Revenue’s Calendar of key dates for tax professionals lists Corporation Tax returns and balances for accounting periods ending 1–31 December 2025 in the 1–23 September 2026 electronic window. As of Monday 24 August — the first working day after the 23 August ROS cash cluster for July PAYE (where the extension applied), July RCT, and November-year CT windows — December year-end companies have thirty calendar days to the 23 September ROS filing and payment date.
Revenue’s corporation tax 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 ROS. Non-electronic filers use a shorter 1–21 September window.
What must clear by 23 September
Form CT1, Form 46G (Company) where required, and iXBRL-tagged financial statements where tagging applies — treat validation failures as filing blockers, not cosmetic warnings.
Any corporation tax balance due on the same date as the return. Preliminary tax for December year-ends should already have been settled; September is the true-up and return pack.
Interest runs on late or incomplete payments, and late returns attract surcharges plus possible restriction of loss and group relief claims — diary the ROS upload, not only the bank payment.
Thirty-day prep checklist
With the August ROS cash cluster behind you, lock management accounts and book-to-tax adjustments so iXBRL tagging is not waiting on open July journals.
Confirm ROS digital certificates and signing access for the company inbox before mid-September holidays thin coverage.
Gather Form 46G contractor and third-party payment details beside the CT1 computation — not as a last-day surprise.
Keep September PAYE (14/23), bi-monthly VAT (19 September), and this CT1 window on one sequenced ROS calendar.
How FinnAccountings helps before the CT1 ROS upload
Clean books shorten the CT1 review cycle. FinnAccountings helps Ireland and UK limited companies prepare bookkeeping and tax packs with Chartered Accountant insight before you or your Chartered Accountant (Ireland) or ICAEW/ACCA adviser files. Start a free trial for review-ready drafts — we prepare records; we do not file your CT1 through ROS or sign off corporation tax returns on your behalf.
Related reading
Yesterday’s thirty-one-day CT1 countdown still maps surcharge and preliminary-tax context.
This morning’s September kickoff still sequences PAYE, VAT, and CT1 after the 23 August cluster.
Bi-monthly July–August VAT shares the September ROS month — keep both packs sequenced.
Use the dual-market tax calculator for high-level planning estimates only — not a CT1 computation.
Sources & references
This article draws on official guidance from the sources below.
- Calendar of key dates for tax professionals
Revenue (Irish Tax and Customs) · Accessed 2026-08-24 - Corporation Tax (CT) payment and filing
Revenue (Irish Tax and Customs) · Accessed 2026-08-24 - When is preliminary Corporation Tax (CT) due?
Revenue (Irish Tax and Customs) · Accessed 2026-08-24
Put this advice into action
FinnAccountings automates bookkeeping, tax, and VAT for Ireland and the UK.
Try AI accounting free