The Uí Briúin were an Irish

Ireland

Ireland is an island to the northwest of continental Europe. It is the third-largest island in Europe and the twentieth-largest island on Earth...

 kin-group. Their eponym

Eponym

An eponym is the name of a person or thing, whether real or fictitious, after which a particular place, tribe, era, discovery, or other item is named or thought to be named...

ous apical ancestor was Brion

Brion (Irish)

Brión , son of Eochaid Mugmedón, was a legendary and possibly historical Irish king, fl. 4th/5th century.-Biography:The older half-brother of Niall of the Nine Hostages and one of the three brothers whose descendants were known as the Connachta, Brión is said to have been king of Connacht....

, son of Eochaid Mugmedon

Eochaid Mugmedon

-Biography:According to medieval Irish legend and historical tradition, Eochaid was a High King of Ireland, best known as the father of Niall of the Nine Hostages and ancestor of the Uí Néill and Connachta dynasties...

 and Mongfind

Mongfind

Queen Mongfind was the wife, of apparent Munster origins, of the legendary Irish High King Eochaid Mugmedón and mother of his eldest three sons, Brion, Ailill and Fiachrae, ancestors of the historical Connachta, through whom she is an ancestor of many Irish and European nobility today...

, and an elder half brother of Niall of the Nine Hostages

Niall of the Nine Hostages

Niall Noígíallach , or in English, Niall of the Nine Hostages, son of Eochaid Mugmedón, was an Irish king, the eponymous ancestor of the Uí Néill kindred who dominated Ireland from the 6th century to the 10th century...

. They formed part of the Connachta

Connachta

The Connachta are a group of medieval Irish dynasties who claimed descent from the legendary High King Conn Cétchathach...

, along with the Uí Fiachrach

Uí Fiachrach

The Uí Fiachrach were a dynasty who originated in, and whose descendants later ruled, the coicead or fifth of Connacht at different times from the mid-first millennium onwards. They claimed descent from Fiachrae, an older half-brother of Niall Noigiallach or Niall of the Nine Hostages...

 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
  Cruachan, Ireland
  Rathcroghan is a complex of archaeological sites near Tulsk in County Roscommon, Ireland. It is identified as the site of Cruachan, the traditional capital of the Connachta. While it is debatable whether this was a place of residence, it had huge importance as a cemetery and also hosted some of...
  
   in modern County Roscommon
  County Roscommon
  County Roscommon is a county in Ireland. It is located in the West Region and is also part of the province of Connacht. It is named after the town of Roscommon. Roscommon County Council is the local authority for the county...
  
  . Major divisions of the Uí Briúin Ai were the Síol Muireadaigh
  Síol Muireadaigh
  Síol Muireadaigh, Gaelic-Irish dynasty and territory, located in north County Roscommon.-Overview:The Síol Muireadaigh were a dynasty of related clans, all descendants of King Muiredach Muillethan of Connacht , all of whom lived in north-central Connaught...
  
  , from whom the many high medieval dynasties of Ua Conchobair
  O'Conor Don
  The Ó Conchubhair Donn is the hereditary Prince and Chief of the Name of the Royal Family of Connacht, the Clan Ó Conchubhair.-Overview:...
  
   (O'Connors), as well as the MacDermot
  MacDermot
  Mac Diarmata is an Irish surname, and the surname of the ruling dynasty of Moylurg, a kingdom that existed in Connacht from the 10th to 16th centuries.-Overview:...
  
  s, were descended, and the Síl Cathail.

• The Uí Briúin Bréifne, whose high medieval kingdom of Bréifne lay in modern County Cavan
  County Cavan
  County Cavan is a county in Ireland. It is part of the Border Region and is also located in the province of Ulster. It is named after the town of Cavan. Cavan County Council is the local authority for the county...
  
   and County Leitrim
  County Leitrim
  County Leitrim is a county in Ireland. It is located in the West Region and is also part of the province of Connacht. It is named after the village of Leitrim. Leitrim County Council is the local authority for the county...
  
  . The O'Reilly and Ó Ruairc dynasties were among the septs of the Uí Briúin Bréifne.

• The Uí Briúin Seóla, who were centred around Tuam
  Tuam
  Tuam is a town in County Galway, Ireland. The name is pronounced choo-um . It is situated west of the midlands of Ireland, and north of Galway city.-History:...
  
   in modern County Galway
  County Galway
  County Galway is a county in Ireland. It is located in the West Region and is also part of the province of Connacht. It is named after the city of Galway. Galway County Council is the local authority for the county. There are several strongly Irish-speaking areas in the west of the county...
  
  .

• The Uí Briúin Umaill, in far west Connacht, on the shores of Clew Bay
  Clew Bay
  Clew Bay is a natural ocean bay in County Mayo, Ireland. It contains Ireland's best example of sunken drumlins. According to tradition, there is an island in the bay for every day of the year. The bay is overlooked by Croagh Patrick, Ireland's holy mountain, and the mountains of North Mayo. Clare...
  
  , modern County Mayo
  County Mayo
  County Mayo is a county in Ireland. It is located in the West Region and is also part of the province of Connacht. It is named after the village of Mayo, which is now generally known as Mayo Abbey. Mayo County Council is the local authority for the county. The population of the county is 130,552...
  
  .

According to Tirechan, Patrick visited the "halls of the sons of Brion" at Duma Selchae in Mag nAi, but does not give their names. An equilvant passage in the Vita Tripartita, possibly of 9th-centry origin, names six sons. "A series of later sources daing 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").