Northern Ireland is not a nation. Ireland is a nation with four provinces: Ulster, Leinster, Munster and Connaught. Six of Ulster's nine counties are part of the UK, and the other three are part of the Repulblic of Ireland.
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.)
So yes, North Ireland represents a nation seperate to the English, the Welsh, or the Scotts.