If you need to file an income tax return by 31 January 2018 and you have done nothing yet, then the first thing you need is a Unique Taxpayer Reference if you don’t have one already. This is a 10-digit number in a format like 12345 67890 and it is not the same thing as your National Insurance Number.
An accountant can apply for one for you at once, but it may take ten days to arrive. The accountant can immediately give you an Application Reference Number, and after a couple of days you can telephone the Revenue on 0300 200 3300 to see if they will give you it over the phone, which they often do. The ARN has a format like A1B2 C3D4 E5F6 GHJ and you only use it once.
With the UTR, a tax return can be prepared and filed electronically at once. You will then need to give the accountant permission to be able to go online and check it, which is another stage that the accountant will deal with.
There is a considerable risk that the deadline will be missed now that things have been left to this late stage. The penalty is 100 pounds, but you would do well to consider the next deadline on 2 March 2018. If you miss this then there is a penalty of 5% of the tax due. This is the deadline that we would target, being realistic.