Cormac was regarded as a saintly figure after his death, and his shrine at Castledermot, County Kildare, was said to be the site of miracles. He was reputed to be a great scholar, being credited with the authorship of the Sanas Cormaic (Cormac's Glossary), and the now-lost Psalter of Cashel, among other works. The reliability of some of the traditions concerning Cormac is doubtful.
The Ireland of Cormac's time was divided into a small kingdoms, or túatha, perhaps 150 in all, on average around 500 square kilometres in area with a population of some 3000. In theory, but not in practice, each tuath had its own king, bishop and court. Variations in size and power were very considerable. Groups of tuatha were dominated by one of their number, whose king was their collective ruler. Above these stood the four great provincial kingships whose names survive in the provinces of Ireland of today: Connacht, Leinster, Ulster and Cormac's Munster. To these can be added the kings of the northern and southern Uí Néill. These last provided were the High Kings of Ireland, kings whose authority was an increasingly obvious political fact in Ireland of the 8th and 9th centuries. In Cormac's time the High Kingship was held by Flann Sinna of the Clann Cholmáin branch of the southern Uí Néill. In addition to these native Irish kings, Ireland, during the Viking Age, had seen Scandinavian and Norse-Gael kings establish themselves along the coasts. The destruction of Viking settlements on the northern coasts by Flann's predecessor Áed Findliath, followed by a much internal dissension, had weakened the Vikings, who were expelled from Dublin by Flann's allies in the year that Cormac became king in Munster.
Cormac belonged to a minor branch of the Eóganachta kindred which dominated Munster in the 8th and 9th centuries. According to genealogies, he was a member of the Eóganacht Chaisil, the Cashel branch of the kindred. This kin group was important, but Cormac came from a very minor branch. He was reckoned an 11th generation descendant of Óengus mac Nad Froích and none of his ancestors since Óengus were counted as kings of Cashel. For this reason, it is most likely thought that Cormac, as well as other 9th century kings of Munster who were bishops and abbots, was a compromise candidate.
The Annals of the Four Masters, a 17th century compilation of annals based on earlier works, but including much of uncertain reliability, say that Cormac was tutored by Snerdgus of Dísert Díarmata (now Castledermot). Some later accounts claim that Cormac had been married or betrothed to Gormlaith, daughter of Flann Sinna, the High King of Ireland, but instead took vows of celibacy. Russell suggests these are later fictions and Byrne sees in them an echo of earlier tales of the sovereignty goddess. Although there is no doubt that Cormac was a bishop before and while he was king of Munster, it is not clear which see Cormac held. Some writers have suggested that he should be linked with Emly rather than Cashel.
Cormac may have attempted to restore the authority of the kings of Munster over neighbouring Leinster and perhaps aspired to be chief king in Ireland. The surviving record, written largely from a northern and pro-Uí Néill perspective, presents a misleading picture, seriously understating the power and pretensions of the Eóganachta. So it is that in 907 only the southern Annals of Innisfallen report campaigns by Cormac in Connacht and Mide, where Flann Sinna was defeated, and record a fleet operating on the River Shannon on his orders which captured Clonmacnoise.
After the army of Munster had gathered, while riding through the camp, Flaithbertach mac Inmainén's horse stumbled and threw him to the ground. This was taken to be a very bad omen. Many of the Munstermen were unwilling to fight, and news of this came to Cerball mac Muirecáin, who proposed a negotiated settlement. The Leinstermen would pay tribute, and give hostages, but the hostages would be given to Móenachem abbot of Dísert Díarmata, rather than to the Munstermen. Cormac was willing to accept this settlement, but Flaithbertach—Byrne notes that later traditions make Flaithbertach Cormac's evil genius— was not and persuaded Cormac to fight, in spite of the king's conviction that he would be killed.
This, and the news than Flann and the Uí Néill had come to Cerball's aid, led to desertions from Cormac's army, but he marched on Leinster all the same, meeting Cerball and Flann at Bellach Mugna (Bellaghmoon, in the south of modern County Kildare). The Fragmentary Annals say that "the men of Munster came to the battle weak and in disorder" and they quickly broke and fled the field. Many were killed. Cormac was among them, after his neck was broken from falling off his horse. Flaithbertach was captured.
Cormac was beheaded and his head taken to Flann Sinna. The Fragmentary Annals say:
"That is indeed evil," said Flann to them, and it was not thanks that he gave them. "It was an evil deed," he said, "to cut off the holy bishop's head; I shall honour it, and not crush it." Flann took the head in his hands, and kissed it, and he carried the consecrated head and the true martyr around him three times.
Following Cormac's death, Munster was seemingly without a king for some years until Flaithbertach mac Inmainén was chosen, apparently another compromise candidate.
That Cormac was reckoned to be a saint in the 11th century is attested by contemporary evidence. The Fragmentary Annals of Ireland state that Cormac was buried at Dísert Díarmata where he was honoured, and add that "Cormac's body ... produces omens and miracles".
The Fragmentary Annals are equally glowing in their praise of Cormac's scholarship and piety: "A scholar in Irish and in Latin, the wholly pious and pure chief bishop, miraculous in chastity and in prayer, a sage in government, in all wisdom, knowledge and science, a sage of poetry and learning, chief of charity and every virtue; a wise man in teaching, high king of the two provinces of all Munster in his time.
A variety of works have been associated with Cormac. The Sanas Cormaic, a glossary of difficult words in Irish in the style of Isidore of Seville, bears his name. While the core of the document dates from around Cormac's time, and may in some way be linked to him, it is far from certain that he was the compiler of even the original list. The lost Psalter of Cashel is also linked to Cormac, as is the Lebor na Cert, the Book of Rights. The works that survive today are probably from the time of Muirchertach Ua Briain.