The guy who lives next to my parents has all his cars on NI plates last few years , no issues; In Ireland you don't deregister your car so if customs do say anything just put your Irsh reg back on; you don't have to pay VRT again.
If you are only driving her the odd time like me; I say the chances of getting pulled is fairly remote. I got the pack from the DVLA with all the forms you need (Pack is free) , think it is around 25 gbp to register her.
The only reservations I have is , Insurance, I am paying 650 euro full comp down here; talking to the guys up in NI insurance can be as high as 1000 gbp + ; this negates the saving in Motor tax plus the risk factor for only saving a few hundred is not worth it.
If I did have to put her back on Southern plates, I am just afraid the insurance companies down here will not re-insure me at a reasonable premium; I am sure you know what they are like.
My friend in NI just bought a house so I was waiting till he bought something before I could look into Insurance prices (needed the post code) ; but this was the show stopper for me. I heard anywhere in around Belfast area it is a nightmare to get insurance
If pulled my argument for Customs is I have my second car (Polo 1000cc) on Southern plates ; I just bring down the Supra from NI the odd time. I know they do take you PPSI number and verify if you work down south. Good luck trying to argue have a Supra on NI plates if you live in Cork . I am practicing my NI accent
But Colin in SSI(Supra Specialists Ireland) will get her MOT ready for you if you decide to go down this route