Major dynasty of medieval Ireland, which long dominated Ulster.
It was founded by a shadowy figure known as Niall of the Nine Hostages (r. 379–405). Divided into northern and southern branches, the Uí Néill ruled as high kings, to whom all other Irish kings owed deference. In the early 11th century the king of Munster, Brian Boru , challenged the high kings of the Uí Néill dynasty and ended their dominance.