From Fr. Thomas Walsh's Ecclesiastical History of Ireland c. xx, p. 191-194
The see of Cashel was founded in the tenth century by Cormac MacCulinan in whose person the mitre and sceptre were united. Before the time of this prelate, Cashel, though the residence of the kings of Munster, was subject to the jurisdiction of the see of Emly. In the council of Kells held in 1152 under Cardinal Paparo, Cashel was raised to the archiepiscopal dignity and its prelate invested with the pallium it then ranked next to Armagh and St. Malachy desired to have this mark of distinction which the pallium conferred bestowed on this see as well as Armagh. Cormac erected a cathedral in Cashel which according to the annals of the island of All Saints was afterwards rebuilt and consecrated with great solemnity.
Between the founder Cormac and the council of Kells only four of his successors are mentioned their acts and those of other eminent ecclesiastics having been completely destroyed. Cormac MacCulinan was born in the year 837 and was of the Eugenian branch being lineally descended from Aengus who had been baptized by St Patrick. Having received an ecclesiastical education under Snegdus the erudite abbot of Castle Dermot, he was admitted to holy orders and it seems was after a certain period raised to the episcopacy on account of his extraordinary merit as it was customary in the Irish Church to promote persons who were distinguished to that rank in places where previously no bishops presided.
Some conjecture that he had been bishop of Lismore before his removal to Cashel as a Cormac son of Culinan is stated to have been bishop of that see in those times nor is it easy to admit two bishops sons of Culinan as contemporaries at Cashel and Lismore.
Kingeagan who then swayed the sceptre of the province having incurred the displeasure of his subjects was deposed from that throne of which he obtained possession in the year 895 and Cormac was called to occupy it by his opponents in 901. Some time however elapsed before Cormac obtained quiet possession of the sovereignty. In 902 when Feongaine son of Gorman king of Cashel was slain in a contest by his own people, Cormac the year after obtained the throne without opposition and thus the spiritual and temporal authority were united in the person of Cormac nor was such an union unknown in his days as Olchobair who died in 851 and Cinfeled who departed life in 872 were kings of Munster and bishops of Emly...
Unhappily at this period a martial spirit pervaded the Irish clergy which was so contrary to the feelings of their predecessors it originated in the contests with the Danes and in which the clergy were of necessity involved as they were frequently obliged to take up arms and defend themselves and their establishments against those savage and ruthless invaders. A desperate battle was fought in which Cormac was slain by one Fiacha a herdsman and along with him the abbots MacEogan and Colman, Kelly the prince of Ossory, Fogarty prince of Kerry and about six thousand of Cormac's forces were put to the sword. Before the engagement Cormac made his confession to Comghall and also prepared his will in which he bequeathed sacred ornaments and utensils gold and silver to various churches and religious places, Cashel, Lismore, Emly, Armagh, Kildare, Glendaloch, Inniscathy and Mungairid. It is said that his body was conveyed to Cashel and there interred According to another account he was buried at Castledermot. This prelate was the author of the celebrated work the Psalter of Cashel in which the ancient events of Irish history are chronologically treated. He also compiled an Irish glossary and a work on the genealogies of the Irish saints. Cormac erected a small but beautiful chapel on the summit of the rock in the city of Cashel. Cormac MacCullenan governed the province of Munster for the space of seven years and during his reign acquired the character of a just and learned prince. Fortune smiled upon him his power was dreaded and by his subjects he was revered because of his many virtues.