More directly, Northern Ireland is where the king of the English nation once sent the peoples of England to colonize the lands of the Irish nation. These people

More directly, Northern Ireland is where the king of the English nation once sent the peoples of England to colonize the lands of the Irish nation. These peoples thus settled have remained culturally distinct as the favored elites of the ruling colonial power, and are presently the majority in the area, which is why the area did not join with greater Ireland during Irish independence. The people of Northern Ireland today tend to want either continued union with the kingdom ruling Great Britain, or union with the rest of Ireland — depending whether their families are of British origin or Irish. The thought of post-Brexit passport control and customs for travel between the territories comprising Northern Ireland and either Great Britain or Ireland is thus a major political football (far worse in the case of the border with Ireland, because it is a land border and that requires walls.)