There has been a anomaly for years in Northern Ireland. Cars there do not have the blue stripe with stars and GB - just a plain number plate containing either Z or I as part of reg. Like the rest of UK, rear plates are yellow. Republic of Ireland has white plates front and back with the blue stripe and IRL.
We recently took a ferry from Dublin to France, and there were a number of NI cars on the trip. Most of them had no country identifier, apart from a few with IRL sticker, which is incorrect.
I suspect that some in NI won’t be rushing to get UK stickers - the current number plates with no country identifier are probably a political compromise.