Donnchad mac Cellaig (also called Donnchadh Ramhor, or the Fat) reigned as king of Osraige from AD 934 to 976.
He was the son of Cellach mac Cerbaill, king of Osraige (died c. 908) and his wife, Echrad ingen Matudán.
He succeeded to the throne on the death of his brother Cuilen, and ruled his native territory with great distinction and credit to himself for more than 40 years.
Osraige rendered hostages to Muircheartach mac Neill, prince of Aileach, who was brother-in-law to Donnachad by his sister Dubhdara.
By his zeal all the principal churches in Ossory were enabled to dispense by the privileges of the Apostles, very liberal charity to the poor: great numbers of his friends also took orphans and other poor, under their protection; every house in Ossory was obliged by his orders to have three vessels, in one of which each inmate deposited a tenth of his food; in another was kept the portion of the poor commonly called Mihal [i.e. Mir Michil, portio Sancti Michaelis]; and in the third which may be called crematha, were kept the crumbs and fragments which were specially under the care of the mistress of the house.