Fionn mac Cumhaill, also known as Finn McCool or Finn MacCool, was an Irish folk hero, who led the a band of hunter-warriors called the Fianna.
Finn was the posthumous son of Cumhall, leader of the Fianna by Muirne.Cumhall abducted Muirne after her father refused him her hand, so Tadg appealed to the high king Conn, who outlawed Cumhall. The Battle of Cnucha was fought between Conn and Cumhall, and Cumhall was killed by Goll mac Morna, who took over leadership of the Fianna.
Muirne was already pregnant; her father rejected her and ordered his people to burn her, but Conn would not allow it and put her under the protection of Fiacal mac Conchinn, whose wife, Bodhmall the druid, was Cumhall's sister. In Fiacal's house Muirne gave birth to a son, whom she called Deimne, literally "sureness" or "certainty", also a name that means a young male deer; several legends tell how he gained the name Fionn when his hair turned prematurely white.
Fionn and his brother Tulcha mac Cumhal were being hunted down by the Goll, the sons of Morna, and other men. Consequently, Finn was separated from his mother Muirne, and placed in the care of Bodhmall and the woman Liath Luachra ("Grey of Luachra"), and they brought him up in secret in the forest of Sliabh Bladma, teaching him the arts of war and hunting. After the age of six, Finn learned to hunt, but still had cause to flee from the sons of Morna.
As he grew older he entered the service – incognito – of a number of local kings, but each one, when he recognized Fionn as Cumhal's son, told him to leave, fearing they would be unable to protect him from his enemies.
Young Fionn, still known by the boyhood Demne, met the poet Finn Éces (Finnegas), near the river Boyne and studied under him. Finnegas had spent seven years trying to catch the salmon that lived in Fec's Pool of the Boyne, for it was prophesied the poet would eat this salmon, and "nothing would remain unknown to him". Although this salmon is not specifically called the "Salmon of Knowledge", etc., it is presumed to be so, i.e., the salmon that fed on the nut[s] of knowledge at Segais. Eventually the poet caught it, and told the boy to cook it for him. While he was cooking it, Demne burned his thumb, and instinctively put his thumb in his mouth. This imbued him with the salmon's wisdom, and when Éces saw that he had gained wisdom, he gave the youngster the whole salmon to eat, and gave Demne the new name, Fionn.
Thereafter, whenever he recited the teinm láida with his thumb in his mouth, the knowledge he wished to gain was revealed to him.