If the OP says nothing and just comes to live & work in Ireland, they'll be tax resident here and may subject to income tax here, given that they'd be performing their duties of employment here (unless there's something in the DTA that allows the UK to retain the primary taxing rights - will check this and come back to edit). If their UK employer doesn't know they're here and continues to deduct UK payroll taxes, and the OP pops up on Revenue's radar at any stage, it would be quite a mess for them to unravel.
I know anecdotally of at least one civil servant, originally from a central European country, who went back there for medical treatment while on a sick cert at some point after WFH became the norm, and then kinda just stayed on WFH from there for several weeks...
When it eventually came to light, they were put off pay and their systems access was revoked until they physically presented themselves to resume work.
Edited to add:
Article 18.2 of the Ireland/UK DTA:
"Remuneration or pensions paid out of public funds of the United Kingdom or Northern
Ireland or of the funds of any local authority in the United Kingdom to any individual in
respect of services rendered to the Government of the United Kingdom or Northern Ireland
or a local authority in the United Kingdom in the discharge of functions of a governmental
nature, shall be taxable only in the United Kingdom unless the individual is a national of
Ireland without also being a United Kingdom national."
So your tax position would depend on your role and nationality.