Fionn's birth and early adventures are recounted in the narrative
The Boyhood Deeds of Fionn and other sources. Fionn was the posthumous son of
Cumhall, leader of the
Fianna, by
Muirne. Fionn and his father Cumhall mac Trénmhoir ("son of Trénmór") stem from Leinster, rooted in the tribe of Uí Thairsig ("the Descendants of Tairsiu") There is mention of the Uí Thairsig in the
Lebor Gabála Érenn as one of the three tribes descended from the Fir Bolg. holding the position for 10 years.
Birth 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.
Boyhood Fionn and his brother Tulcha mac Cumhal were being hunted down by 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 recognised Fionn as Cumhal's son, told him to leave, fearing they would be unable to protect him from his enemies.
Thumb of Knowledge Fionn was a keen hunter and often hunted with Na Fianna on the hill of Allen in County Kildare, it is believed by many in the area that Fionn originally caught the Salmon of Knowledge in the River Slate that flows through Ballyteague. The secret to his success thereafter when catching "fish of knowledge" was to always cast from the Ballyteague side of a river. He gained what commentators have called the "Thumb of Knowledge" after eating a certain salmon, thought to be the
Salmon of Knowledge. Young Fionn, still known by his boyhood name 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". Fionn's acquisition of the Thumb of Knowledge has been likened to the Welsh
Gwion Bach tasting the Cauldron of Knowledge, and
Sigurðr Fáfnisbani tasting
Fáfnir's heart.
Fire-breather of the Tuatha de Danann in Violet Russell's
Heroes of the Dawn (1914) One feat of Fionn performed at 10 years of age according to the
Acallam na Senórach was to slay
Áillen (or), the fire-breathing man of the
Tuatha Dé Danann, who had come to wreak destruction on the Irish capital of
Tara every year on the festival of
Samhain for the past 23 years, lulling the city's men to sleep with his music then burning down the city and its treasures. After Fionn defeated Áillen and saved Tara, his heritage was recognised and he was given command of the Fianna: Goll stepped aside, and became a loyal follower of Fionn, although a dispute later broke out between the clans over the pig of Slanga.
Almu as eric Before Fionn completed the feat of defeating the firebrand of the fairy mound and defending Tara, he is described as a ten-year-old "marauder and an outlaw". It is also stated elsewhere that when Fionn grew up to become "capable of committing plunder on everyone who was an enemy", he went to his maternal grandfather Tadg to demand compensation (
éric) for his father's death, on pain of single combat, and Tadg acceded by relinquishing the estate of Almu (the present-day
Hill of Allen). Fionn was also paid
éric by Goll mac Morna. == Adulthood ==