Fergus Dubdétach was, according to medieval Irish legend and historical tradition, a king of the Ulaid

Ulaid

The Ulaid or Ulaidh were a people of early Ireland who gave their name to the modern province of Ulster...

 who was briefly High King of Ireland

High King of Ireland

The High Kings of Ireland were sometimes historical and sometimes legendary figures who had, or who are claimed to have had, lordship over the whole of Ireland. Medieval and early modern Irish literature portrays an almost unbroken sequence of High Kings, ruling from Tara over a hierarchy of...

. He took the High Kingship after his predecessor, Lugaid mac Con

Lugaid mac Con

Lugaid Mac Con, often known simply as Mac Con, was, according to medieval Irish legend and historical tradition, a High King of Ireland. He belonged to the Corcu Loígde, and thus to the Dáirine. His father was Macnia mac Lugdach, and his mother was Sadb ingen Chuinn, daughter of the former High...

, was expelled from Tara

Hill of Tara

The Hill of Tara , located near the River Boyne, is an archaeological complex that runs between Navan and Dunshaughlin in County Meath, Leinster, Ireland...

 by Cormac mac Airt

Cormac mac Airt

Cormac mac Airt , also known as Cormac ua Cuinn or Cormac Ulfada , was, according to medieval Irish legend and historical tradition, a High King of Ireland...

 and killed in Munster by Cormac's poet Ferches mac Commáin. Fergus and his two brothers, Fergus Caisfhiachlach ("rough-tooth") and Fergus Foltlebair ("long-hair"), then expelled Cormac to Connacht

Connacht

Connacht , formerly anglicised as Connaught, is one of the Provinces of Ireland situated in the west of Ireland. In Ancient Ireland, it was one of the fifths ruled by a "king of over-kings" . Following the Norman invasion of Ireland, the ancient kingdoms were shired into a number of counties for...

 and Fergus took the throne. He ruled for a year, before he was defeated by Cormac, with the assistance of Tadg mac Céin and Lugaid Láma, in the Battle of Crinna.

The Lebor Gabála Érenn

Lebor Gabála Érenn

Lebor Gabála Érenn is the Middle Irish title of a loose collection of poems and prose narratives recounting the mythical origins and history of the Irish from the creation of the world down to the Middle Ages...

makes no reference to Fergus' descent. Geoffrey Keating

Geoffrey Keating

Seathrún Céitinn, known in English as Geoffrey Keating, was a 17th century Irish Roman Catholic priest, poet and historian. He was born in County Tipperary c. 1569, and died c. 1644...

 gives him a long genealogy, naming his father as Finnchad, son of Ogaman, son of the former High King and king of the Ulaid Fíatach Finn

Fiatach Finn

Fiatach Finn mac Dáire, a distant descedant of Óengus Tuirmech Temrach, was, according to medieval Irish legend and historical tradition, a king of the Ulaid, later a High King of Ireland, and the eponymous ancestor of the early Medieval Ulster dynasty of the Dál Fiatach...

. The Annals of the Four Masters

Annals of the Four Masters

The Annals of the Kingdom of Ireland or the Annals of the Four Masters are a chronicle of medieval Irish history...

name Fergus' father as Imchad, but traces his descent no further.

The Lebor Gabála synchronises Lugaid's reign with that of the Roman emperor

Roman Emperor

The Roman emperor was the ruler of the Roman State during the imperial period . The Romans had no single term for the office although at any given time, a given title was associated with the emperor...

 Commodus

Commodus

Commodus , was Roman Emperor from 180 to 192. He also ruled as co-emperor with his father Marcus Aurelius from 177 until his father's death in 180. His name changed throughout his reign; see changes of name for earlier and later forms. His accession as emperor was the first time a son had succeeded...

 (180-192). The chronology of Keating's Foras Feasa ar Éirinn dates his reign to 203-204, that of the Annals of the Four Masters to 225-226.