Cormac mac Airt

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:

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. Cormac had a younger brother, Alastir.

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.

...
Wikipedia