Uí Briúin

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 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, 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.

The Uí Briúin divided into multiple septs, the three major ones being:

• The Uí Briúin Aí, named for the region they controlled—Mag nAí, the lands around the ancient centre of Connacht, Cruachan in modern County Roscommon. The most notable sept of the Uí Briúin Ai was the Síol Muireadaigh, from whom the ruling families of Ó Conchubhair (O'Connor) and MacDermots, descended.
• The Uí Briúin Bréifne, whose high medieval kingdom of Bréifne lay in modern County Cavan and County Leitrim. The Ó Ruairc (O'Rourkes) dynasty was the senior sept of the Uí Briúin Bréifne. Other septs included The Ó Raghallaigh (O'Reilly), Mág Tighearnán (McKiernan) and Mág Samhradháin (McGovern).
• The Uí Briúin Seóla, who were centred on Maigh Seóla in modern County Galway. The Ó Flaithbheartaigh kings of Iar Connacht and their kin, the Clann Cosgraigh, belong to this branch.

The Uí Briúin kings of Connacht were drawn exclusively from these three branches.

According to Tírechán, Saint Patrick visited the "halls of the sons of Brión" at Duma Selchae (located by John O'Donovan in Mag nAí and alternatively by Roderic O'Flaherty near Loch Cime), 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 Uí Briúin was responsible for this dramatic swelling of the ranks, as tribes and dynasties newly coming under Uí Briúin sway were furnished with ancestries that would link them genealogically to their overlords. Into this category fall the Uí Briúin Umaill and likely also the Uí Briúin Ratha and Uí Briúin Sinna."