The Uí Briúin were an Irish kin-group. Their eponymous apical ancestor was Brion, 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 Uí Ailello, putative descendants of Eochaid Mugmedon's sons Fiachra and Ailill. The Uí Ailello were later replaced as the third of the Three Connachta, by 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 7th and 8th centuries.
The Uí Briúin divided into multiple septs, the major ones being:
- The Uí Briúin Ai, named for the region they controlled—Mag Ai, the lands around the presumed ancient centre of Connacht, Cruachan in modern County Roscommon. Major divisions of the Uí Briúin Ai were the Síol Muireadaigh, from whom the many high medieval dynasties of Ua Conchobair (O'Connors), as well as the MacDermots, were descended, and the Síl Cathail.
- The Uí Briúin Bréifne, whose high medieval kingdom of Bréifne lay in modern County Cavan and County Leitrim. The O'Reilly and Ó Ruairc dynasties were among the septs of the Uí Briúin Bréifne.
- The Uí Briúin Seóla, who were centred on Annaghdown in modern County Galway.
- The Uí Briúin Umaill, in far west Connacht, on the shores of Clew Bay, modern County Mayo.
According to Tírechán, Saint Patrick visited the "halls of the sons of Brion" at Duma Selchae in Mag nAi, but does not give their names. An equivalent passage in the Vita Tripartita, possibly of 9th-century origin, names six sons. "A series of later sources dating from the eleventh century onward, meanwhile, enumerates Brion's progeny as no less than twenty-four. No doubt the increasing power of the Ui Briuin was responsible for this dramatic swelling of the ranks, as tribes and dynasties newly coming under Ui Briuin sway were furnished with ancestries that would link them genealogically to their overlords. Into this category fall the Ui Bruin Umaill, and likely also the Ui Bruin Ratha and Ui Bruin Sinna." (p485, "Ui Bruin", Anne Connon, in "Medieval Ireland: An Encyclopedia").
- 801. Connmhach, Judge of Ui Briuin, died.