The O'Neill dynasty are a lineage of Irish Gaelic origin that held prominent positions and titles in Ireland and elsewhere. As kings of Cenél nEógain, they were historically one of the most prominent family of the Northern Uí Néill, along with the O'Donnell dynasty. Some O'Neills state that their ancestors were kings of Ailech during the Early Middle Ages, as descendants of Niall of the Nine Hostages.

The Northern Uí Néill was any of several dynasties in north-western medieval Ireland that claimed descent from a common ancestor, Niall of the Nine Hostages. Other dynasties in central and eastern Ireland who also claimed descent from Niall are termed the Southern Uí Néill. The dynasties of the Northern Uí Néill were the Cenél Conaill and Cenél nEógain, named after the two most powerful sons of Niall: Conall and Eógain.