They have to buy plates in the north! I was looking at two cars before I got mine and they where both on Private plates that they wanted to keep! MAde no difference to me as I was going to bring mine in but would have had to wait two to three weeks before I could get either one as they had to rerigister them!
If you want one in Ireland ie. 555 or 571 somewhere in the number it'll cost you around 350 but you cann't bring it with you!