Irish Corporation Tax Compliance for the 2026 Filing Season

Corporation tax compliance in Ireland has never been a one-date job. Between the CT1 return itself, preliminary tax instalments, and the iXBRL tagging that has to sit behind the accounts, there are several deadlines running in parallel for every accounting period — and 2026 brings a genuine change to how Revenue handles one of them. If you're managing a portfolio of corporate clients, or you're the in-house accountant keeping your own company compliant, this is a good moment to walk back through the mechanics before the autumn filing season builds up.

When is your CT1 actually due?

The standard rule is nine months after the end of the accounting period. But almost every company files through ROS, and for electronic filers that nine-month window is tightened to a fixed date: the 23rd day of the ninth month after the accounting period end. So a company with a 31 December year end has until 23 September to file its CT1 and pay any balance of tax due, while a 30 June year end pushes that out to 23 March. It's a small distinction, but it trips people up every year — "nine months" and "the 23rd of month nine" aren't quite the same date, and Revenue's own guidance sets out the mechanics clearly on the payment and filing page.

Preliminary tax: get the instalment structure right

Preliminary tax is where a lot of the planning work actually happens, and the rules differ depending on the size of the company.

• Small companies — where the prior year's CT liability was below €200,000 — pay in a single instalment, equal to 100% of the prior year's liability (or, at the company's option, 100% of the current year's liability). This is due 31 days before the end of the accounting period, and in any case no later than the 23rd of that month.
• Large companies — prior-year liability at or above €200,000 — generally pay in two instalments where the accounting period runs longer than seven months: the first by the 23rd of month six, the second by the 23rd of month eleven, totalling 90% of the final liability between them. Where the accounting period is seven months or shorter, a single instalment covering 90% of the final liability applies instead.

Revenue sets these thresholds and instalment mechanics out in detail on its preliminary tax guidance, and it's worth every practice keeping a simple instalment calendar per client rather than relying on memory — the "large company" threshold catches more clients each year as profits recover.

iXBRL: the taxonomy behind the numbers

iXBRL tagging of financial statements has been mandatory since 2013/2014 for most companies filing a CT1, and it remains one of the more mechanical-feeling but genuinely important parts of the process — Revenue uses the tagged data to risk-assess returns, so accuracy matters beyond simply "getting it submitted". A deferral from the requirement is available, but only where all three of the following apply: balance sheet total not exceeding €4.4 million, turnover not exceeding €8.8 million, and average employee numbers not exceeding 50. Miss any one of the three thresholds and the exemption doesn't apply.

The taxonomy itself — the technical schema that defines how each line item is tagged — is updated periodically by Revenue, and the specific version in force, along with any changes to the size thresholds, should always be checked against the current Tax and Duty Manual (Part 41A-03-01) rather than assumed from a previous filing season. It's a small habit, but it prevents a rejected submission at the worst possible moment.

The 2026 change that raises the stakes

From 1 January 2026, Revenue is withdrawing the facility that allowed companies to file draft, unsigned iXBRL accounts where the audited or finalised set wasn't ready in time. Chartered Accountants Ireland flagged this as a meaningful practical shift: where a company genuinely can't finalise its accounts before the CT1 deadline, the route now is to contact the local Revenue branch directly, not to submit a placeholder set of draft figures. In practice, this means director sign-off needs to happen earlier in the process than some practices are used to — leaving iXBRL tagging and finalisation until the final week before the CT1 deadline is a riskier habit than it used to be.

What else changed for 2026

A handful of other measures from Budget 2026 and Finance Act 2025 are worth having on the radar for corporation tax clients this season:

• R&D tax credit increase. Following Budget 2026, the R&D tax credit rate rises from 30% to 35%, and the threshold for the accelerated first-year payable/refund element increases from €75,000 to €87,500 — a meaningful boost for companies with qualifying R&D spend, though the exact commencement detail is best confirmed against Revenue's published guidance as it's finalised.
• Participation exemption for foreign dividends. Introduced in 2024 and broadened under Finance Act 2025, the exemption now extends relief to a wider range of territories and shortens the "excluded acquisition" lookback period from five years to three — worth revisiting for any group with overseas subsidiaries.
• Pillar Two registration. For large multinational groups with consolidated global revenue above €750 million, Ireland extended the Pillar Two registration deadline to 28 February 2026 for in-scope groups whose first Pillar Two fiscal year ended in 2024. Missing registration risks a €10,000 penalty, and the substantive GloBE Information Return filing generally falls due 15 months after fiscal year-end (18 months for a transition year).

Late filing: what's actually at stake

Missing the CT1 deadline isn't just a paperwork issue — it triggers a surcharge on top of the tax due, broadly in the region of 5% of the tax liability (subject to a cap) where the return is filed within two months of the deadline, rising to broadly 10% (subject to a higher cap) where it's filed later than that, with daily interest running on any unpaid tax throughout. The exact percentage caps are set out in Revenue's Tax and Duty Manual and are worth checking directly before advising a client on exposure, since these figures are reviewed periodically. Beyond the direct cost, a late CT1 can also restrict a company's ability to claim certain reliefs and can affect its tax clearance status — reasons enough to build the filing calendar around the ROS deadline rather than the headline nine-month rule.

Frequently asked questions

Does a company with no trading activity still need to file a CT1?

Generally yes — a company registered for corporation tax is expected to file a CT1 for each accounting period, even a dormant one, unless it has formally deregistered. Check the company's specific registration status with Revenue if you're unsure.

What happens if the accounts genuinely aren't ready in time for iXBRL tagging?

Since the draft-accounts facility is being withdrawn from 1 January 2026, the appropriate step is to contact the company's local Revenue branch directly to explain the difficulty, rather than submitting an unsigned draft set as a stopgap.

Is the preliminary tax instalment structure the same for every company?

No — it depends on the prior year's CT liability. Companies below the €200,000 threshold pay a single instalment; those at or above it generally pay in two instalments (or one, for shorter accounting periods), each calculated as a percentage of the final liability.

Corporation tax compliance rewards a calendar-first approach: mapping the CT1 deadline, the preliminary tax instalment dates, and the iXBRL sign-off point against each client's specific accounting period end, well before the filing season gets busy. For practitioners who want to keep their technical knowledge current across corporation tax, iXBRL, and the wider compliance calendar, Learnsignal's CPD courses for accountants cover exactly this kind of practical, up-to-date filing knowledge alongside the broader technical update requirements professional bodies expect each year.