Flaithbertach (d. 765), son of Loingsech and king of Tara, belonged to Cenél Conaill, a dynasty of the Uí Néill federation, which by the eighth century (if not earlier) had divided into Northern and Southern groupings. His father Loingsech (qv) had earlier reigned as king of Tara. One version of the ‘Banshenchas’ (lore of women) names his mother as Muirenn, daughter of the powerful Leinster overking Cellach Cualann (qv), making him a half-brother of one of his principal rivals – Cináed Cáech (qv) son of Írgalach (qv), whose dynasty of Síl nÁedo Sláine belonged to the Southern Uí Néill grouping, but this account is probably the result of a scribal error. In any event, Flaithbertach had at least four recorded brothers, three of whom (Artgal, Connachtach, and Flann Gerg) were slain along with their father Loingsech in 704, when Flaithbertach was still a child. The surviving brother, Fergal, led Cenél Conaill while their father's cousin, Congal Cennmagair (qv), was king of Tara. In 707 Fergal assisted Fergal (qv) son of Máel-dúin of Cenél nÉogain in overthrowing and slaying Indrechtach, overking of Connacht.
Precisely when Flaithbertach gained control of Cenél Conaill is not clear; apparently, it was prior to 722. In that year, when Fergal son of Máel-dúin was slain, the Annals of Inisfallen note that Flaithbertach reigned over the Uí Néill. Presumably, he was already prominent among the emerging Northern Uí Néill; it may even be that by that time the kingship of Ailech was already established on a firm basis. It appears, however, that the prestigious kingship of Tara rested for the next six years with Síl nÁedo Sláine. It may be assumed that Flaithbertach had married prior to this, although the name of his wife (or wives) is not recorded. He had at least three sons – Murchad, Máel-Bresail, and Áed Muinderg – and a daughter, Dúnlaith, who married Niall Frossach (qv) of Cenél nÉogain and was the mother of the latter's son Áed Oirdnide (qv).
If indeed Flaithbertach dominated the Northern Uí Néill by the 720s, he already faced strong opposition from Áed Allán (qv) of Cenél nÉogain. For a time, he was able to stall Áed, defeating him at Druim Fornocht in 727. This victory enabled him to stake his claim to the kingship of Tara, which at this stage was held by Cináed Cáech. In 728, Flaithbertach managed to defeat and kill Cináed, whereupon, in the words of partisan annalists, he ‘took the kingship of Ireland’. The regnal list in the Book of Leinster credits him with seven years as king of Tara, but it appears, however, that from 732 his authority within the north began to unravel. In that year, Flaithbertach's forces suffered a serious reverse after an onslaught by their Cenél nÉogain rivals. One of his leading supporters, his second-cousin, Flann Goan son of Congal, was slain. The following year, Áed Allán invaded the Cenél Conaill territory of Mag nÍtha (a plain to the south of Inishowen) and another cousin, Conaing son of Congal, fell while defending Flaithbertach's rights. In an effort to retaliate against Cenél nÉogain, Flaithbertach hired the fleet of Dál Riata from Scotland. This brought no advantage, as many of his newly-recruited auxiliaries perished in a disastrous naval engagement at the mouth of the River Bann. In 734 he again faced an invasion of Mag nÍtha. On this occasion, the battle was in effect a standoff, with both sides suffering heavy casualties. At this point, however, Flaithbertach – who was clearly discouraged – decided to abdicate and leave the kingship of Tara to his Cenél nÉogain rivals.
Flaithbertach was the last Cenél Conaill ruler to hold the paramount Uí Néill kingship; indeed, the loss of Mag nÍtha to Cenél nÉogain meant that for two centuries his dynasty was marginalised within the politics of the north-west. Following his abdication, Flaithbertach retired to a religious life and died at Armagh thirty years later. His son Murchad, who reigned as local king of Cenél Conaill, was slain in 767. Through his other sons, Máel-Bresail and Áed Muinderg, Flaithbertach was the common ancestor of the family lines of Ua Máele-Doraid and Ua Canannáin, which provided several of the kings of Cenél Conaill in the later medieval period.