Northern Ireland is inhabited by the Ulster Scots. Many of these people, although lowlanders by origin, have tartans and clan associations.
A caveat needs to be added to this statement, though: the original Ulster-Scots who came from the Lowlands would not have had tartans and clan associations; Lowland and Border families have adopted tartans and formed "clan" societies, but only after the "tartan craze" of the 19th century. When the plantation of Ulster was first settled, Lowlanders and Bordereres would have held Highlanders and anything associated with them in about as low regard as they did the "wild" Irish.

As Scott mentioned, you do have some Highlanders that settle in Ulster, though. You also have English, Welsh, and even some contientials, such as the German Palantines and French Protestants.

Regards,

Todd