The thought had occurred to me, but personally speaking (and going into rather murky waters as a sometimes controversial issue) I wouldn't count either Northern Ireland in particular or Ireland in general as a British colony, given that prior to the Act of Union it was, at least on paper, independent, and that after it Ireland received the same representation given any other part of the United Kingdom. If you're going to count Ireland as a British colony then you'd have to count, say, Croatia as an Austrian colony or Poland as a Russian colony (and while those were without a doubt occupied territories they're not usually termed colonies)
I think you can legitimately count Northern Ireland as a colony that meets the terms of the OP, given that more than half the population of the province are the descendants of colonists.