Customs are on the ball these days, a few months back they came into the estate with a couple of flat bed trucks and knocked on a few doors; lifted a few cars on NI plates.
But some guys seem to get away with it , it is all down to luck! I was going to do it this year but was talked out of it. I might get away with it for a few days or a few months/years but until you hit a checkpoint , then you are on there radar. Revenue check against your PRSI number where you work; trying to argue you live up north and commute is going to be a challenge.
I can use Steven's house but he is a few hours away in Derry so likely hood I commute is very slim, but I was going to use my second car (Polo) to say I use it down south (its on South plates) and argue I bring down the Supra the odd Day. But it is just a bit messy with having to change clocks over to MPH etc for the MOT and Customs; considering what the car is I reckon they will make it there mission to catch me out.
On the plus side down here, you don't de-register a car so I can still keep my southern plates, I would not have to pay VRT again. If they did force me to put her back on Irish plates.
It is an issue with Stolen cars , car is scrapped ; people keep the reg, rob an equivalent Model/Make /Color/ Year and start driving her under the scrapped cars plates. AFAIK they don't check the VIN plate at NCT time (MOT) easy money!