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, formerly also spelled Connaught (/ˈkɒnɔːt, ˈkɒnə(x)t/, Irish: Connachta [ˈkʊn̪ˠəxt̪ˠə] or Cúige Chonnacht [ˌkuːɟə ˈxʊn̪ˠəxt̪ˠ]), is the smallest and least populous of the four provinces of Ireland, situated in the west of Ireland. Until the ninth century it consisted of several independent major Gaelic kingdoms (Uí Fiachrach, Uí Briúin, Uí Maine, Conmhaícne, and Delbhna).
Between the reigns of Conchobar mac Taidg Mór (died 882) and his descendant, Aedh mac Ruaidri Ó Conchobair (reigned 1228–33), it became a kingdom under the rule of the Uí Briúin Aí dynasty, whose ruling sept adopted the surname Ua Conchobair. At its greatest extent, it incorporated the often independent Kingdom of Breifne, as well as vassalage from the lordships of western Mide and west Leinster. Two of its greatest kings, Tairrdelbach Ua Conchobair (1088–1156) and his son Ruaidri Ua Conchobair (c. 1115–1198) greatly expanded the kingdom's dominance, so much so that both became High King of Ireland.
Conchobar mac Taidg Mór (died 882) was a King of Connacht from the Uí Briúin branch of the Connachta. He was the grandson of Muirgius mac Tommaltaig (died 815), a previous king. His father Tadg Mór (died 810) had been slain fighting in Muirgius' wars versus the minor tribes of Connacht. He was of the Síl Muiredaig sept of the Uí Briúin. The Ó Conchobhair septs of Connacht are named for him.