Fáilbe Flann (d. 637/9), son of Áed Dub and overking of Munster, belonged to the Éoganacht dynasty of Cashel. His father may have been a local ruler, but seems not to have achieved particular prominence. Neither the name of his mother nor that of his wife survives, but he had at least one brother, Fíngen; two sons, Colgu and Fogartach; and a daughter Faílenn who married Máel-ochtraig son of Díneartach, a king of the Déisi. While his brother Fíngen (d. 619) apparently reigned as king – perhaps in opposition to Áed Bennán (qv) – Fáilbe's immediate predecessor was Cathal (d. 628) son of Áed Fland Cathrach of the rival lineage of Éoganacht Glendamnach. It is difficult to reconcile this apparent terminus post quem for Fáilbe's accession with the Munster kinglist, which credits him with a reign of twenty years: perhaps he held a regional kingship in Cathal's time.
Fáilbe had already achieved prominence by 627. In that year he soundly defeated the overking of Connacht, Guaire Aidni (qv), at the battle of Carn Feradaig (Cahernarry, Co. Limerick), where the casualties included kings of Uí Maine and Uí Fhiachrach in mid and north Connacht. The result was a consolidation of Munster Déisi settlement in the area of the present Co. Clare south of Slieve Aughty, a district that became known as In Déis Tuaiscirt – and ultimately as Dál Cais. Presumably claims (expressed in the Rawl. B 502 genealogies) that Fáilbe gained the kingship of Cashel because Dál Cais relinquished an ancient right to share the Munster overkingship represent a later interpretation of these developments. The marriage of Fáilbe's daughter Fáilenn to a Déisi dynast should probably be viewed in the same context.
An ambitious ruler, Fáilbe sought to extend the influence of Cashel in southern Ireland. He patronised the emerging Leinster dynasty of Uí Dúnlainge, forming an alliance with its ruler Fáelán (qv) son of Colmán Már. According to the annals, he supported Fáelán at the battle of Áth Goan (west of the river Liffey, probably in Kildare), where the Uí Chennselaig king of Leinster, Crimthann Cualann (qv), was slain. This may be the historical reality behind later Munster claims that he paid the Bóruma tribute on behalf of Leinster. On his death in 637 (or 639), Fáilbe Flann was succeeded by the relatively undistinguished Cúán son of Amalgaid, a member of the Éoganacht Áine. Later the kingship passed to his son Colgu (d.678), from whom descended the later rulers of Clann Fáilbe.