The UàBriúin were a royal dynasty of Connacht. Their ous 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 UàFiachrach and UàAilello, putative descendants of Eochaid Mugmedon's sons Fiachra and Ailill. The UàAilello were later replaced as the third of the Three Connachta, through genealogical sleight of hand, by the UàMaine.
Connacht was ruled in early times by the UàFiachrach, the UàBriúin only becoming the dominant force in Connacht in the 7th and 8th centuries.
The UàBriúin divided into multiple septs, the three major ones being:
The UàBriúin kings of Connacht were drawn exclusively from these three branches.
According to TÃÂrechán, Saint Patrick visited the "halls of the sons of Brión" at Duma Selchae (located by John O'Donovan in Mag nAàand 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 UàBriúin was responsible for this dramatic swelling of the ranks, as tribes and dynasties newly coming under UàBriúin sway were furnished with ancestries that would link them genealogically to their overlords. Into this category fall the UàBriúin Umaill and likely also the UàBriúin Ratha and UàBriúin Sinna."
While Francis Byrne and John O'Donovan believed the dynasty originated in Mag nAÃÂ, Roderic O'Flaherty and John Colgan related traditions of Saints Patrick and Felartus visiting the sons of Brión in Maigh Seóla. This confusion surrounding the location of Mag Selce as mentioned in the Tripartite caused Nicholls to suggest that the geographical origin of the UàBriúin was moved for political reasons to near Cruachan by the time of TÃÂrechán. MacCotter also points out that when the UàBriúin were purportedly beginning their ascent, given the distributions of surrounding population groups, "the area originally available to UàBriúin [in Mag nAÃÂ] cannot have consisted of more than the area of a few civil parishes", which may lend support to Nicholls' theory.
A story in the Silva Gadelica notes that during the legendary war between Brión and Fiachra, Fiachra's encampment was situated in Aidhne and Brión's lay in Damh-Chluain, which is stated to be in UàBriúin Seóla and not far from Knockma Hill, west of Tuam. Although this is a legend, it may be an indication of the UàBriúin's original homeland, as is Aidhne for the UàFiachrach. In addition, Hubert Knox, citing the Conmaicne's distribution and early status as subjects of the UàBriúin, posited that the UàBriúin originated in the barony of Clare in County Galway as the leading lineage of that people. Intriguingly, the Book of Ballymote calls Cellach mac Rogallaig "King of Conmaicne", a title also commonly taken by members of the UàBriúin Bréifne branch in later centuries. The Maigh Seóla origin scenario is more consistent with the fact that early UàBriúin kings (e.g. Cenn Fáelad mac Colgan and Cellach mac Rogallaig) had their residence on Loch Cime, as well as ÃÂed mac Echach's donation of Annaghdown in the 6th century, which Byrne thought unlikely given Annaghdown's distance from Mag nAÃÂ. Furthermore, Cenn Fáelad mac Colgan is stated in the annals as having been killed by the Conmaicne Cuile, and the king-list in Laud 610 states that the same king died at the hands of "his own people". If both of these accounts are accurate, it would indicate that the UàBriúin originated among the Conmaicne.
With the inclusion of Máenach mac BáethÃÂne, ancestors of all three major branches of the dynasty are mentioned in the annals as residing or fighting in the Maigh Seóla region in the 7th century. The district to the east of Lough Corrib and the River Corrib is referred to as "Magh Ua mBriuin" at least as late as 1149. This likely denotes the domain of the "king of UàBriúin", a title borne primarily by men of the UàBriúin Seóla. As Knox points out, these kings were distinguished at an early date from the SÃÂol Muireadaigh of central Roscommon in the Book of Rights, suggesting that the lands of the UàBriúin Seóla were the original "Hy Briuin".
A recent study found that the Y-DNA SNP A259 is likely the defining mutation for the dynasty. While all of the branches share a common ancestor in the timeframe of Brión, there are no extant genealogies which correctly detail the first generations. Notably, the UàBriúin Bréifne are more closely related to the UàBriúin Seóla than they are to the UàBriúin AÃÂ.