this name derives from the irish surname “Ó riagáin”, meaning "son of riagán", from the irish personal name “riagán”, meaning "little king". the o'regans of meath were a branch of the southern ui neill and one of the four tribes of tara. before the anglo-norman invasion, they were lords of south breagh in meath and the north of the present co. dublin, and appparently a powerful family. they took a leading part in the wars against the danes. the annalists, under the year 1029, record a notable triumph of mathghamhain Ó riagáin, king of breagh, over the foreigners, when he made prisoner amhlaoibh, son of sitric, king of dublin, and only released him on payment of an enormous ransom, including the celebrated sword of carlus. regan is a city in burleigh county, north dakota, united states.