In very simple terms - Northern Ireland is roughly divided equally between Protestants who largely identify as British/with the United Kingdom (Unionists) and Catholics who largely identify as Irish (Nationalists). There are several complicating elements to this basic outline which will have some readers of this post twitching, but that’s the broad dynamic at play. When the region was carved out in the early 1920s the demographic balance was considerably more in favor of the Unionists, but Catholic birth-rates since then have changed things.
The accent is distinct, but this is not unusual on the island, as accents vary a lot throughout.
The south is heavily (90+%) Catholic, but it’s not an issue as southern Protestants identify as Irish too. So in the event of a United Ireland, Unionists would go from holding an equal share in a UK territory with which they identify (and which they have historically dominated politically, legally, economically and culturally) to a minority in a state with which they do not identify at all.