Dictionary of National Biography, 1885-1900/Ceallachan
CEALLACHAN (d. 954), king of Cashel, called in poetry C. coir, or the just, and c. cruaidh, or the hard, is the hero of several old popular tales of Munster. He was king of Cashel from 935 till his death in 954. He first appears in history as plundering Clonmacnoise in 935, and in 937 ravaged Meath in alliance with the Danes of Waterford. In 939 he ravaged Ossory and the Decies, but later in the same year was defeated by their tribes. Muircheartach, king of Ailech, invaded the south early in 941, and carried off Ceallachan as a hostage to Donegal, where he kept him for nine months, and then sent him to Donnchadh, king of Ireland, who set him free. In 942 Ceallachan defeated Cenneide, father of Brian Boroimhe, in the battle of Maghduin, and ever after ruled in comparative quiet till his death from natural causes in 954. Ceallachan was chief of the great tribe called the Eoghanacht, and is the ancestor of many families once powerful in the south of Ireland. The O'Ceallachans or O'Callaghans of the south take their name from the great-grandson of his son Donnchadh, and the last chief in direct line of the chief branch of his race is believed to have been Donnchadh (or Denis) O'Callaghan of Glinn, who died in 1760, having married his cousin Mary O'Callaghan in 1745, and left one daughter of the same name. Cornelius, her kinsman, though in what degree is not known, was in 1785 created Baron Lismore in the peerage of Ireland.
[Chronicon Scotorum (Rolls Series), p. 201; Tracts relating to Ireland (Irish Archæolog. Soc. 1841), pp. 43, &c.; Annala Rioghachta Eireann, vol. ii.; genealogical manuscripts of the late B. C. Fisher.]