One of the most tragical occurrences of his reign was the murder of thirty princesses by Dunlaing, king of Leinster, in the house known as the southern Claenfert at Tara. Cormac quickly avenged their deaths by slaying twelve chieftains of Leinster, and imposing the tax called the Boruma on Leinster with increased severity. This tax had originally been exacted by Tuathal Teachtmhar (A.D. 106), and was a perennial source of warfare between the Leinster rulers and their overking. It was finally remitted through the intervention of St. Dairchell [q. v.]
Towards the close of his reign occurred the expulsion of the Desi, descendants of Fiacha Suighdhe, brother of Conn of the Hundred Battles, who were seated in the plain of Breagh. According to one account of the cause of this event, Aengus, ‘of the dreadful spear,’ or, as ‘Lebar na h-Uidhre’ has it, ‘the poisoned spear,’ having been wronged by Cellach, son of Cormac, hastened in a fury to Tara, slew Cellach in his father's presence, killing also the steward of Tara, and piercing his father's eye by the same stroke that killed his son. For this crime the tribe of the Desi, to which Aengus belonged, were expelled by Cormac after several battles, and finally settled in Waterford, where they have given their name to the baronies of Decies.
To the reign of Cormac belongs the history of the famous warrior Finn mac Cumhail, who was slain, according to the ‘Four Masters,’ in 283. The only unsuccessful battle in which Cormac was engaged was that of Droma Damgaire, now Knocklong, in the county of Limerick. Cormac had made an unprovoked attack on Fiacha Muilleathan, king of Munster, assigning as a pretext that a double tribute was due to him as overking, inasmuch as there were two provinces in Munster. Receiving a reply that there was no precedent for such a demand, he marched direct for Droma Damgaire, and a battle ensued in which he was defeated and pursued to Ossory, and also obliged to give hostages and indemnify Fiacha for his losses. Neither the ‘Four Masters’ nor Tigernach make any special mention of this expedition, though minute accounts of it are preserved in the ‘Book of Lismore’ and elsewhere. ‘The truth is’ (as Dr. O'Donovan observes) ‘that the annalists of Leath Cuinn (the north of Ireland) pass over the affairs of Munster very slightly, and seem unwilling to acknowledge any triumph of theirs over the race of Conn of the Hundred Battles, and this feeling was mutual on the part of the race of Olioll Olum.’
The injury to Cormac's eye already referred to made it necessary for him, according to Irish custom, to abdicate, as no one with a personal blemish could reign at Tara. He was accordingly succeeded by his son, and retired to Aicill, now the hill of Skreen, near Tara, visiting occasionally Cleiteach on the Boyne. He now applied himself to legislation, and his reputation in this capacity far exceeded his martial achievements. ‘He was a famous author in laws, synchronisms, and history; for it was he that established law, rule, and direction for each science and for each covenant according to propriety, and it is his laws that governed all that adhered to them to the present time’ (Four Masters).
Dr. Petrie, in his ‘Essay on the History and Antiquities of Tara Hill,’ discusses at some length the question of the laws attributed to him. On the subject of the use of letters in Ireland at that early period, which affects the authenticity of Cormac's alleged legislation, Innes observes: ‘It may have very well happened that some of the Irish before that time passing over to Britain or other parts of the Roman empire where the use of letters was common might have learned to read and write.’
Cormac is said to have become a Christian seven years before his death, being ‘the third man in Ireland who believed.’ This will appear possible when it is considered that he had been in contact with Roman civilisation in Britain, where Christianity is known to have spread among the Roman colonists about the commencement of the third century (Haddan). He died at Cleiteach, A.D. 260. The early account simply says he was choked by a salmon bone; but an interlined gloss in ‘Lebar na h-Uidhre’ suggests that it was the siabhra or genii that killed him, and the ‘Four Masters’ add that it was on account of his abandoning the worship of idols. The account of his burial seems to favour the belief that he was a Christian. It is said in ‘Lebar na h-Uidhre’ that he desired to be buried at Ros na righ, but after his death it was decided that he should be interred at Brugh na Boinne, ‘where all the kings of Tara were buried.’ When, however, they proceeded to carry out their purpose, the river Boyne ‘rose against them three times,’ and they had to abandon the attempt, and he was taken to Ros na righ, which was thenceforward the burial-place of the Christian kings. The reign of Cormac is the epoch at which most of the monuments remaining at Tara had their origin. Of these an interesting account will be found in the learned essay of Dr. Petrie.
