[[Íomhá:Ireland early peoples and politics.gif|mion|Luath dhreamanna agus rochtain in Éirinn, c. 800]]
''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:
* Uí Bhriúin Aoi (''Aí''), named for the region they controlled — Mag nAí, the lands around the ancient centre of Connacht, [[
Cruachan, Ireland|Cruachan]] in modern [[ County Roscommon]]. Major divisions of the Uí Briúin Ai were the [[Síol Muireadhaigh]], from whom the many high medieval dynasties of [[Uí Chonchubhair]], as well as the [[Mhic Dhiarmada]] s, were descended, and the [[Síol Chathail]].
* ''Uí Bhriúin Bhréifne, whose high medieval kingdom of [[Bréifne]] lay in modern [[County Cavan]] and [[County Leitrim]]. The [[Uí Raghallaigh]] and [[Uí Ruairc]] dynasties were among the septs of the Uí Briúin Bréifne. Other septs included Mag Tighearnán and Mag Samhradháin.
* Uí Bhriúin
Seola, who were centred on [[ Maigh Seola]] in modern [[ County Galway]]. The [[Uí Flaithbheartaigh]] kings of [[ Iar Connacht]] and their kin, the [[Clann Chosgraigh]], 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."<ref>Anne Connon, "Uí Briúin" in ''Medieval Ireland: An Encyclopedia'', lch. 485.</ref>