Some nonsense being spoken on here.
NI cars are UK spec cars and the registration documents come from Swansea. They are MOTd and taxed in exactly the same way apart from the fact that MOT stations are government run.
loads of ‘uk’ reg cars over here as well. The single difference is the NI reg. think of it as a crap private plate for free.
All this talk of imports and resale values will have the DUP walking out for another couple of years.
RoI cars will have KPH speedometers and require importing but that it totally different.
ETA: resale values of some models can be higher in NI due to the smaller market and cost of travelling to other parts of the UK. What the OP is contemplating often happens in reverse.