The Uí Briúin were a royal dynasty of Connacht. Their eponymous apical ancestor was Brión, son of Eochaid Mugmedon and Mongfind, and an elder half brother of Niall of the Nine Hostages. They formed part of the Connachta, along with the Uí Fiachrach and , putative descendants of Eochaid Mugmedon's sons Fiachra and Ailill. The Uí Ailello were later replaced as the third of the Three Connachta, through genealogical sleight of hand, by the Uí Maine. Connacht was ruled in early times by the Uí Fiachrach, the Uí Briúin only becoming the dominant force in Connacht in the 7th and 8th centuries.