There are a few key dates for all businesses you should be aware of.
Corporation Tax deadline - you need to file a Corporation Tax Return each year 8 months and 23 days after the year end.
Corporation Tax payments - you need to pay the first chunk of Corporation Tax for the year on the 23rd say of the last month and has to pay either 100% of last years bill, or 90% of this years bill if you know what that is. For a December year end, the Preliminary Tax is due on 23rd November. You also need to pay the balance due from last years bill, if any.
Self Employed and company directors
Tax year end - this is always 31st December for self employed people and company directors
Income Tax filing date - 31st October each year or mid November if you pay and file online ( or your accountant does )
Income Tax payment - pay 100% of last years bill for Preliminary Tax on 31st October each year or mid November if you file online. Therefore in October / November 2018 you are paying 100% of the 2017 bill towards your 2018 bill, and the balance due for 2017 if there is a shortfall in Preliminary Tax for 2017.
These dates are important as Revenue will bring their powers to bear on your if you are late in filing:
Interest for late filing and payment - 5% penalty for late filing up to the first month, 10% penalty after that. Interest is charged at 15% per annum on late payments. You are also more likely to have a Revenue audit if you are late with your filings or payment. It's important to keep up to date with your bookkeeping so that you are not caught unawares by these deadlines.