Cellach (d. 776), son of Dúnchad and overking of Leinster, belonged to the dynasty of Uí Dúnlainge. His father Dúnchad (qv) (d. 728), ancestor of the lineage of Uí Dúnchada, briefly enjoyed the overkingship of Leinster, but Cellach's immediate predecessor was his uncle Muiredach, progenitor of the rival Uí Muiredaig lineage. There is no record of Cellach's wife (or wives), but he had at least four sons – Fínshnechtae Cetharderc (qv), Fáelán (d. 804), Muiredach (d. 822), and Áed (d. 828) – and a daughter, Muirenn (d. 830). While his reign could justifiably be described as quiet, it was not entirely devoid of dynastic strife: in the battle of Áth Orc (770) he slew the king of Uí Fhailge, Cináed son of Flann dá Chongal (qv), and his ally the king of Fothairt. An opportunity such as that provided by internal dissension within Leinster was not lost on the dynasties of Uí Néill, which throughout most of the eighth and ninth centuries were seeking to extend their authority. That same year, the king of Tara, Donnchad Midi (qv) son of Domnall Midi (qv) invaded the province; he encamped at Dún Ailinne (Knockaulin, Co. Kildare) and ravaged the surrounding countryside.
Cellach's reign certainly witnessed a reassertion of the dynasty's interests in ecclesiastical affairs. For instance, his aunt Condal (qv) was abbess of Kildare. In addition, three of his sons in turn, Fáelán, Muiredach, and Áed, and his daughter Muirenn, also held senior ecclesiastical posts there; this, however, might owe more to his son and ultimate successor, Fínshnechtae Cetharderc. A late addition in the Uí Dúnlainge genealogies in the Book of Leinster credits Cellach with granting the site of Tallaght to St Máel-ruain (qv) and with being a major benefactor of Glendalough. There are strong indications that Uí Dúnlainge interests at Glendalough, if not a product of Cellach's reign, date to the latter decades of the eighth century. The immediate successors of Cellach were his cousins Ruaidrí and Bran Ardchenn (qv), before the kingship reverted to his son Fínshnechtae.