Fiachu mac Néill (flourished 507–514)[1] was a king of Uisnech in Mide of the Ui Neill dynasty.
[4] Fiachu appears as the conqueror of Meath in the annals with Failge Berraide, the ancestor of the Leinster dynasty of Uí Failgi, as his opponent.
In 507 Fiachu was defeated by Failge at the Battle of Frémainn (Frewin Hill, near Mullingar, County Westmeath).
[6] Fiachu was ancestor of the Cenél Fiachach, a clan which included several well known sub-clans or septs such as Geoghegan and O'Higgins, whose lands extended from Birr to Uisnech in southern Westmeath and part of north Offaly and their southern territory became known as Fir Cell (land of the churches), and later the Barony of Moycashel.
Another son Crimthann was great-grandfather of a local saint Áed mac Bricc (died 589).