Flaithbertach mac Inmainén (died 944) was abbot of Inis Cathaig (Scattery Island) and sometime King of Munster in the south of Ireland.
Flaithbertach is first mentioned by the Irish annals in 907, when he is recorded, along with the then-King of Munster Cormac mac Cuilennáin, leading an expedition by the Munstermen against Connacht and the Uí Néill.
According to the partisan pro-Munster Annals of Innisfallen, Cormac and Flaithbertach defeated Flann Sinna, the High King of Ireland, and later obtained hostages from the Uí Néill.
[1] In 908, Cormac and Flaithbertach collected an army to campaign against their eastern neighbours, Leinster, whose king Cerball mac Muirecáin was Flann Sinna's son-in-law and staunch ally.
The Fragmentary Annals say that the clerics of Leinster, apparently led by the abbess of Kildare, Muirenn ingen Suairt, subjected Flaithbertach to harsh criticism for his part in the death of the saintly Cormac: "The evil things that certain scholars of Leinster said about Flaithbertach are shameful to tell, and improper to write.