Fogartach may be identified with the "Focortoch" who signed as a guarantor of the Cáin Adomnáin at Birr in 697. The earliest report of him in the Irish annals is his flight from the battlefield at the Battle of Claenath in 704 following the defeat of a number of southern Uí Néill kings by Cellach Cualann, King of Leinster. In 714, Fogartach was deposed as king of Brega and exiled in Britain. It has been suggested that it was the High King, Fergal mac Máele Dúin, who deposed him, but it appears more likely that this was a dispute within the fractious Síl nÁedo Sláine, and that Fogartach was removed by his uncle Conall Grant, assisted by Murchad Midi of Clann Cholmáin. Conall killed Murchad the following year and Fogartach returned in 716. He caused some manner of disturbance in 717 at the Oenach Tailtiu—an annual Uí Néill gathering held at Teltown—where "Ruba's son and Dub Sléibe's son" were killed, but the annalistic record lacks sufficient context to explain what happened there and why. The following year Conall Grant won a battle against a coalition of southern Uí Néill kings at Kells, but was killed by Fergal mac Máele Dúin later that year. In the early 720s, Fogartach's lands were under attack by the kings of Leinster and Cathal mac Finguine, king of Munster. Fergal mac Máele Dúin undertook campaigns against Leinster in revenge, but was killed by the Leinstermen on one of these, at the battle of Allen, on 11 December 722. His brother Áed Laigin was slain in this battle.
Fogartach replaced Fergal as High King, but himself fell victim to the war within the Síl nÁedo Sláine, being killed in the battle of Cenn Deilgden by his distant kinsman and successor Cináed mac Írgalaig of the Uí Chonaing sept of North Brega. This was an old feud, Cináed's father had assassinated Fogartach's father in 701. The report of his death in the Annals of Ulster does not refer to him as High King.
His descendants representing the main line of the Uí Chernaig sept based at Lagore were in rivalry with his uncle Conall Grant's descendants, the Síl Conaill Graint based at Calatruim for the rule of southern Brega.
