Ireland CT Preliminary Tax: Nine Days to the 23 August Window for September Year-Ends

Revenue’s calendar of key dates for tax professionals lists Corporation Tax preliminary tax for accounting periods ending between 1 and 30 September 2026 in the 1–23 August electronic window. The same window covers CT returns and balances for accounting periods ending 1–30 November 2025. Non-electronic filers use 1–21 rather than 1–23.

As of 14 August you have nine days to the ROS close. Sequence this beside today’s PAYE/DWT/PSWT cluster, the 19 August VAT 3 date, and the 23 August RCT July return so September year-end companies do not miss preliminary tax while chasing mid-month payroll and VAT packs.

Small vs large company preliminary tax

A small company — prior-period Corporation Tax liability not above €200,000 (annualised for short periods) — generally makes one preliminary tax payment of either 100% of the previous period’s liability or 90% of the current period’s liability (with top-up rules where needed). New start-ups below that €200,000 threshold typically skip preliminary tax for their first accounting period and pay the final charge with the CT1.

A large company — prior-period liability above €200,000 — generally pays in two instalments (around month six and again in the month before the period ends). Group members may apply to Revenue’s Debt Management Task Force for notional allocation of payments across the group.

For a September 2026 year-end, the calendar’s August window is the payment date that sits one month before the accounting period ends (21st / 23rd electronic of that month). Confirm whether your company is on the single-payment or two-instalment track before you authorise ROS.

November 2025 CT1 balances in the same window

Companies with accounting periods ending 1–30 November 2025 also see return filing and balance payments listed for 1–23 August 2026. That is the nine-month CT1 deadline measured to the 23rd for electronic filers. Keep iXBRL accounts and Form 46G on the same checklist if those packs are still open.

Do not confuse this August CT window with the separate 23 September 2026 CT1 date for December 2025 year-ends — different accounting periods, different diaries.

Nine-day ROS checklist

Confirm each Irish company’s accounting period end and small/large status against last year’s CT1. Calculate the preliminary tax base (prior-year 100% or current-year 90%) and diary the ROS payment method so cash clears by 23 August.

Open ROS → Payment and Returns → Corporation Tax → make a payment for the correct period. If the period is missing, use MyEnquiries to open it before paying — misapplied payments can bounce as refunds.

Keep management accounts, tax computations, and bank evidence export-ready for your adviser. Pair CT cash planning with July VAT 3 (19 August) and RCT (23 August) so one ROS session does not crowd out another.

How FinnAccountings helps with CT packs

Organised ledgers and period closes make Corporation Tax preliminary estimates easier to review. FinnAccountings helps Ireland and UK businesses 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 CT1 or pay Revenue on your behalf.

Start a free trial →

Related reading

December year-end companies should keep the separate 23 September CT1 countdown on their diary.

Ireland CT1 countdown: 23 September 2026 →

For the full August SME diary including PAYE, VAT 3, and RCT, use the Ireland August deadlines checklist.

Ireland August 2026 tax deadlines for SMEs →

Principal contractors still need the July RCT deduction summary verified before 23 August.

Ireland RCT: ten days to 23 August →

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

Open free tax calculator (Ireland & UK) →