Fergus Dubdétach ("black-tooth") was, according to medieval Irish legend and historical tradition, a king of the

Ulaid Ulaid (Old Irish, ) or Ulaidh ( Modern Irish, ) was a Gaelic over-kingdom in north-eastern Ireland during the Middle Ages made up of a confederation of dynastic groups. Alternative names include Ulidia, which is the Latin form of Ulaid, and in ...

who was briefly

High King of Ireland High King of Ireland ( ga, Ardrí na hÉireann ) was a royal title in Gaelic Ireland held by those who had, or who are claimed to have had, lordship over all of Ireland. The title was held by historical kings and later sometimes assigned an ...

. He took the High Kingship after his predecessor, Lugaid mac Con, was expelled from Tara by Cormac mac Airt 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 ( ; ga, Connachta or ), is one of the provinces of Ireland, 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 ...

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'' (literally "The Book of the Taking of Ireland"), known in English as ''The Book of Invasions'', is a collection of poems and prose narratives in the Irish language intended to be a history of Ireland and the Irish fro ...

'' makes no reference to Fergus' descent. Geoffrey Keating 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 mac Dáire, a distant descendant 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 Ulste ...

. The ''

Annals of the Four Masters The ''Annals of the Kingdom of Ireland'' ( ga, Annála Ríoghachta Éireann) or the ''Annals of the Four Masters'' (''Annála na gCeithre Máistrí'') are chronicles of medieval Irish history. The entries span from the Deluge, dated as 2,24 ...

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

'
M226
/ref> The ''Lebor Gabála'' synchronises Lugaid's reign with that of the Roman emperor

Commodus Commodus (; 31 August 161 – 31 December 192) was a Roman emperor who ruled from 177 to 192. He served jointly with his father Marcus Aurelius from 176 until the latter's death in 180, and thereafter he reigned alone until his assassination. ...

(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.

References

Legendary High Kings of Ireland {{Ireland-royal-stub