Hello and welcome to the site. Thank you for your question.
Firstly, you should register for self assessment on the grounds that you are in receipt of income from UK property. You can register for SA online and the link is here
As you live in Ireland, you are regarded as non-resident landlord. If you wish for rental income to be paid to you without deduction of tax at source you need to fill in form NRL1i and once your application is appoved by HMRC, you will receive rent gross and declare it on your self assessment tax return. More information on this can be found here
You have stated you have an apartment in Liverpool that has been rented for some time but you wish to deaclare rental income from now.
You should be mindful that no disclosure of rental income may result in penalty/fine and interest on tax due not paid. It is in your interest to prepare property income accounts from day one and see what the net profit was each year you had rental income.
You would be able to claim your personal allowance against it being a UK citizen and the tax may not be high.
I hope this is helpful and answers your question.
If you have any other questions, please ask me before you rate my service – I’ll be happy to respond.