Cormac
mac Airt (son of Art), also known as Cormac ua Cuinn (grandson
of Conn) or Cormac Ulfada (long beard), was, according to medieval
Irish legend and historical tradition, a High King of Ireland.
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 said to have ruled from Tara, the seat of the High Kings
of Ireland, for forty years, and under his rule Tara flourished.
He was famous for his wise, true, and generous judgments. In
the Annals of Clonmacnoise, translated in 1627, he is described
as:
"…absolutely the best king that ever reigned
in Ireland before himself...wise learned, valiant and mild,
not given causelessly to be bloody as many of his ancestors
were, he reigned majestically and magnificently".
The hero Fionn mac Cumhaill is supposed to have lived in Cormac's
time, and most of the stories of the Fenian Cycle are set during
his reign.
Cormac's
father was the former High King Art mac Cuinn. His mother was
Achtan, daughter of Olc Acha, a smith (or Druid) from Connacht.
According to the saga "The Battle of Mag Mucrama",
Olc gave Art hospitality the night before the Battle of Maigh
Mucruimhe. 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. (Geoffrey
Keating says that Achtan was Art's official mistress, to whom
he had given a dowry of cattle).
The story is told that Achtan had a vision as she slept next
to Art. She saw herself with her head cut off and a great tree
growing out of her neck. Its branches spread all over Ireland,
until the sea rose and overwhelmed it. Another tree grew from
the roots of the first, but the wind blew it down. At that she
woke up and told Art what she had seen. Art explained that the
head of every woman is her husband, and that she would lose
her husband in battle the next day. The first tree was their
son, who would be king over all Ireland, and the sea that overwhelmed
it was a fish-bone that he would die choking on. The second
tree was his son, Cairbre Lifechair, who would be king after
him, and the wind that blew him down was a battle against the
Fianna, in which he would fall. The following day Art was defeated
and killed by his nephew Lugaid mac Con, who became the new
High King.
In a story reminiscent of the Roman myth of Romulus and Remus,
Cormac was carried off in infancy by a she-wolf and reared with
her cubs, but a hunter found him and brought him back to his
mother. Achtan then took him to Fiachrae Cassán, who
had been Art's foster-father. On the way they were attacked
by wolves, but wild horses protected them.
Cormac
owned a wonderful gold cup given to him by the sea-god Manannan
mac Lir in the Land of the Living. If three lies were spoken
over it, it would break in three; three truths made it whole
again. Cormac used this cup during his kingship to distinguish
falsehood from truth. When Cormac died, the cup vanished, just
as Manannan had predicted it would.

Cormac mac
Airt is asked by his grandson Carbre "what were your habits
when
you were a lad?" Cormac replies as follows:
I
was a listener in woods,
I was a gazer at stars,
I was blind where secrets were concerned,
I was silent in a wilderness,
I was talkative among many,
I was mild in the mead-hall,
I was stern in battle,
I was ready to watch,
I was gentle in friendship,
I was a physician of the sick,
I was weak towards the strengthless,
I was strong toward the powerful,
I never was hard lest I be satirised,
I never was feeble lest I should have my hair stripped off,
I was not close lest I should be burdensome,
I was not arrogant though I was wise,
I was not given to promising though I was strong,
I was not venturesome, though I was swift,
I did not deride old people, though I was young,
I was not boastful though I was a good fighter,
I would not speak about anyone in his absence,
I would not reproach, but I would praise,
I would not ask, but I would give,
For it is through these habits that the young
become old and kingly warriors.

Sources:
Instructions of Cormac, § 7
The Battle of Mag Mucrama (translator unknown)
Geoffrey Keating, Foras Feasa ar Éirinn
Francis J. Byrne, Irish Kings and High Kings, Four
Courts press, 2001, p. 65-69
Mairin O Daly (ed.), The Heroic Biography of Cormac mac
Airt, Cath Maige Mucrama : the battle of Mag Mucrama,
Irish Texts Society, 1975.
Standish Hayes O'Grady (ed. & trans.), The Panegyric
of Cormac mac Airt, Silva Gadelica, 1892