UK driver wants to drive Irish car in Ireland for extended periods

I'm aware that UK drivers can drive in Ireland for normal holiday periods. I also know that UK drivers could swap their licenses for Irish ones (at least up to Brexit). I've a different situation, and I don't think either of those applies.

My sibling is domiciled in the UK. They also own a house in Ireland, which they visit for up to a year at a time. They have two cars in the UK and a UK driving license, which they need and cannot give up / exchange. On recent visits to Ireland they have hired a rental car, for months at a time. Now they are considering either: a) importing one of theirs car to Ireland and re-registering, or b) buying a new car in Ireland.

Apart from the complexity of importing, in either case they will have to register, tax, and insure a car in Ireland. Can they do all this on a UK driving license? Is there a time limit on driving in Ireland on a UK license after which it is no longer considered a "holiday". Might they have to do an Irish driving test and get an Irish license. Can you even do that while domiciled in the UK? From what I can tell you don't need any license details to tax a car. Insurance might be more tricky.

Scratching my head here as I cannot seem to find anything that applies to people domiciled in the UK but spending a large amount of time (possibly more than a year) in Ireland. Any help appreciated.