He is probably the most famous of the ancient High Kings, and may have been an authentic historical figure, although many legends have attached themselves to him, and his reign is variously dated as early as the 2nd century and as late as the 4th.
He is credited for building many of the monuments at the Hill of Tara such as the Banqueting Hall, Cormac's house, and Gráinne's Enclosure, named after his daughter.
It had been prophesied that a great dignity would come from Olc's line, so he offered the High King his daughter to sleep with that night, and Cormac was conceived[2] (Geoffrey Keating says that Achtan was Art's official mistress, to whom he had given a dowry of cattle).
Cormac was carried off in infancy by a she-wolf and reared with her cubs in the caves of Kesh (Keash, Co Sligo), but a hunter found him and brought him back to his mother.
[4] At the age of thirty, armed with his father's sword, Cormac came to Tara, where he met a steward consoling a weeping woman.
[5][6] Other traditions say that Cormac drove Lugaid out by force,[7][8] or that he left Tara because his druids had prophesied he would not live another six months if he stayed.
[3] In all versions he went to his kin in Munster, where the poet Ferches mac Commain killed him with a spear as he stood with his back to a standing stone.
But Cormac was unable to claim the High Kingship, as the king of the Ulaid, Fergus Dubdétach, drove him into Connacht, and took the throne himself.
He turned to Tadg mac Céin, a local nobleman whose father had been killed by Fergus, promising him as much land on the plain of Brega as he could drive his chariot round in a day if he would help him claim the throne.
Tadg routed Fergus's army, and ordered his charioteer to make a circuit of the plain of Brega to include Tara itself.
When the charioteer answered no, Tadg killed him, but before he could complete the circuit himself, Cormac came upon him and ordered physicians to treat his wounds - treatment which took a whole year.
Also according to Keating, Cormac took a second wife, Ciarnait, daughter of the king of the Cruthin, but Eithne, out of jealousy of her beauty, forced her to grind nine measures of grain every day.
He is also said to have compiled the Psalter of Tara, a book containing the chronicles of Irish history, the laws concerning the rents and dues kings were to receive from their subjects, and records of the boundaries of Ireland.
He sent messengers to demand payment, but Fiachu Muillethan, the king of southern Munster, refused, and Cormac prepared for war.
His new druids' magic made the camp impregnable and his warriors unbeatable, dried up all sources of water used by the Munstermen, and nearly drove Fiacha to submission.
[12] In the tale Echtra Cormaic (Lady Gregory, GAFM IV.11 "His Three Calls to Cormac" ) the Irish King is tempted by the sea-god Manannan mac Lir with treasure, specifically a "shining branch having nine apples of red gold," in exchange for his family.
Cormac is led into the Otherworld (Land of Promise) and taught a harsh lesson by Manannán, but in the end, his wife and children are restored to him.
Cormac, having lost an eye, moves into the Tech Cletig on the hill of Achall, as it was against the law for a disfigured king to sit in Tara.