Cellach (d. 705), son of Rogallach and overking of Connacht, belonged to the dynasty of Uí Briúin Aí. He is called Cellach ‘of Loch Cime’ (Lough Hacket, near Headford, Co. Galway), perhaps the location of a family residence. His father Rogallach, who earlier reigned as overking, was a son of Uatu son of Áed Abrat (qv). Cellach had at least two brothers, Fergus (d. 654) and Cathal (d. 680), and in turn had two sons, Domnall and Fergus. When Rogallach was slain (649) by Diarmait Ruanaid (qv) son of Áed Sláine (qv), the overkingship reverted to the rival dynasty of Uí Fhiachrach, with Uí Briúin Aí regaining supremacy only on the death of Fergal Aidni in 696.
It appears that Cellach succeeded at this point; he is styled ‘king of the Connachta’ in the witness-list to the Law of Adomnán (qv), which is thought to have been framed in 697. His low position in the list perhaps reflects Cenél Conaill aspirations in regard to Connacht. There is, however, some confusion in the Middle Irish regnal list, with the reigns of Cellach and one of his nephews, Muiredach Aí (in this source called ‘son of Muirgius’ but elsewhere ‘son of Fergus’), difficult to reconcile. Perhaps the overkingship was disputed, in which case Cellach attained pre-eminence rather belatedly, only with Muiredach's death in 702. He faced a major challenge in 704 when Connacht was invaded by the Cenél Conaill king of Tara, Loingsech (qv) son of Óengus. Cellach, although past his middle years, was probably not as decrepit as a strand of saga woven into the Fragmentary Annals (§158) makes him out to be. He blocked the path of the invaders in Corann (south-central Co. Sligo); details of the battle that followed, with the time and date of initial engagement (6 a.m., Saturday, 12 July), are recorded. Cellach completely routed the invaders: Loingsech himself, his three sons, and several of his sub-kings were numbered among the slain.
Cellach died peacefully in 705, having entered clerical retirement. His immediate successor in the overkingship was Indrechtach (d. 707) son of Dúnchad of Uí Fhiachrach. Cellach's sons Domnall (d. 728) and Fergus (d. 756), and grandsons Flaithrí (abd. 777) and Colla (d. 796), later reigned as overkings. Most later rulers of Uí Briúin, however, descended from his nephew Muiredach, the principal lineage of the dynasty becoming known as Síl Muiredaig.