Immediately after the death of Bran Mut a battle between the Leinstermen and their western neighbours the
kingdom of Osraige is recorded, but Cellach is not associated with this. Cellach was one of the guarantors of the
Cáin Adomnáin (Law of Adomnán) in 697 promulgated at the
Synod of Birr. In 704 he warred with his northern neighbours, the
Uí Néill of
Clann Cholmáin, led by Bodbchath mac Diarmata Déin, and of
Síl nÁedo Sláine, led by
Fogartach mac Néill. The Battle of Claenath, fought near
Clane in modern
County Kildare, was a victory for Cellach, with Bodbchath killed and Fogartach fled. In 707 the Uí Néill
High King of Ireland Congal Cennmagair campaigned in Leinster and obtained Cellach's submission to his authority. In 709, aided by
British allies—usually presumed to be mercenaries, exiled members of the warbands of the British kingdoms conquered by expansionist
Northumbria—he fought a battle at
Serg where his sons Fiachra and Fiannamail were killed.
Serg is unidentified,
Whitley Stokes translates it as "the battle of the Hunt". The
Annals of Tigernach locate it somewhere in the
fortuatha of Leinster, that part of the province east of the
Wicklow Mountains. His adversaries are not named. Cellach's death in 715 is reported without comment by the
Irish annals. ==Family==