Fathadh mac Aonghus

Fathadh mac Aonghus (fl. 10th–11th centuries) was an Irish nobleman.

He is the purported ancestor to the Ó Fathaigh/Fahy family of Uí Maine. The first person known bear the surname would appear to be one Cormac mac Maonach (Cormac Ó Fathaigh) who is listed as his great-grandson of the eight generation. However, if it is correct, Fathadh's grandson, Aonghus, or his great-grandson, Flann, would have been its first users. But Fathadh is given as a son of Aonghus mac Lomán, who lived sometime in the sixth, or even fifth, century, hundreds of years before the introduction of surnames in Ireland.