"Each country has its own distinguishing mark for the purposes of international traffic.
Under international conventions, the distinguishing mark for the United Kingdom is 'GB', standing for the United Kingdom of Great Britain and Northern Ireland.
Although vehicles registered in Northern Ireland or Great Britain are not required to display a GB sticker or symbol when driving in the Republic of Ireland, a GB sticker should be displayed on UK-registered vehicles when they're driven in the rest of the EU or EEA."
As Ulster is not part of Great Britain, why is a UK sticker not appropriate?