PREFACE: I am a terrible writer. Also I do not like the concept of the Tir because it is sort of a horrible racist circlejerk in-universe. YOU HAVE BEEN WARNED.
Your family has always been fiercely patriotic. Since the days of Cu Chulainn they have fought in every major revolution, riot, or civil war over most of the British Isles, sometimes winning, sometimes losing, but always killing. When it came time for you to go to war, though, it wasn't over something petty like politics, or religion, or who's inbred bastard son was closer to their mam. No, it was over something far greater: your home. Those pointy eared sons-a-bitches wanted to steal your country, kick out all the Irishmen and make it their own!
You and your family killed twenty men between you. They had magic, and plenty of it, but you and your folks had something stronger: violent, seething hatred for the Tir and all it stood for. You suppose your grandmother must have been right about your bloodline going back to Chulainn, because you certainly turned into something nasty when they managed to hit you with a bolt. You and your folks couldn't hold out forever, though, and before you knew it you were on your way to a tribunal to be tried and then shot, if not necessarily in that order. You and a couple of family members managed to escape, though, steal a boat and sail to an airport. After that you headed for America, where you and your remaining family set yourselves up in an apartment, all the while swearing to yourselves to one day bring down the Tir and all who supported it, and to avenge all you had lost. Until that day comes, though, you'll be damned if you let those bastards take your home a second time.