His exact identity is not known. As the eponymous ancestor of the Uí Failge he may be associated with Rus Failge, the son of the high king of Ireland, Cathair Mór, said to have lived in the early 2 cd century. This is the ancestor listed in the genealogies, though chronologically impossible. The Book of Leinster king list names an early king Failge Rot mac Cathair. The Uí Failge appear to have had an early leadership among the Laigin tribes. 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.
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). Fiachu had a false prophecy that he would win this battle and desired revenge. In 514 he achieved this revenge by defeating Failge at the Battle of Druim Derg. By this victory the plain of Mide was taken away from the Laigin and Fiachu conquered the territory from Birr to Uisnech in County Westmeath.
- all dates per The Chronology of the Irish Annals, Daniel P. McCarthy
- Byrne, Table 8; Charles-Edwards, Appendix XV
- Byrne, pg.139
- Annals of Ulster AU 510.1; Annals of Tigernach AT 507.1
- AU 516.1, 517.3; AT 514.3
- Annals of Ulster at CELT: Corpus of Electronic Texts at University College Cork
- Annals of Tigernach at CELT: Corpus of Electronic Texts at University College Cork
- Byrne, Francis John (2001), Irish Kings and High-Kings, Dublin: Four Courts Press, ISBN 978-1-85182-196-9
- Charles-Edwards, T. M. (2000), Early Christian Ireland, Cambridge: Cambridge University Press, ISBN 0-521-36395-0
- Revised edition of McCarthy's synchronisms at Trinity College Dublin.