County Clare is a county of Ireland, a subdivision of the European Alliance on planet Earth. The capital town is Ennis.
Kathryn Janeway, when meeting Michael Sullivan of Fair Haven for the first time, told him, she had an aunt and uncle living in County Clare, not far from Fair Haven. This led Sullivan to call her "Katie O'Clare" from then on. (VOY: "Fair Haven")
A short time later, when the inhabitants of Fair Haven began to suspect that something was wrong with the visiting Voyager crew, Michael Sullivan did some checking with friends of his in County Clare and found out that nobody had heard of Katie O'Clare there, whereupon Janeway countered that it was a big county. (VOY: "Spirit Folk")