As the eponymous ancestor of the Uí Failge, he may be the son of the high king of Ireland, Cathair Mór, said to have lived in the early 2 cd century.
This is testified by the early Irish poem Timna Cathaír Máir ("The Testament of Cathair Mór) where Rus Failge is given the succession to his father.
[3] Failge appears as the opponent of Fiachu mac Néill of the Ui Neill, founder of the Cenél Fiachach branch.
In 507 Fiachu was defeated by Failge at the Battle of Frémainn (Frewin Hill, near Mullingar, County Westmeath).
By this victory the plain of Mide was taken away from the Laigin and Fiachu conquered the territory from Birr to Uisnech in what became County Westmeath.