It used to be said it was bad luck to wear a Tartan thats not "yours" Being of Irish descent gives you a massive scope of designs. Black watch is commonly known as a "free" Tartan and can be worn be anyone. some Stuart Tartans have some sectarian issues attached to them and Celtic/Rangers Tartan would single you out for a "kicking" in some parts of the west side. My favourite "Free" Tartan is "Flower of Scotland"

here it is modelled by myself