Cumhall mac Trénmhoir was the leader of the Fianna, bands of young roving hunter-warriors in Ireland.
He fell in love with Muirne, the daughter of Tadg mac Nuadat and the granddaughter of Nuadat, the druid serving Cathair Mór, the High King. When Cumhall asked for her hand in marriage, Tadg refused. He refused all Muirne’s suitors because he had foreseen that his daughter’s marriage would cause him to lose his ancestral seat.
Cumhall was furious. He kidnapped Muirne intending to make her his bride. Tadg appealed to the Conn Cétchathach the new High King who outlawed Cumhall’s action and sent men to pursue him. Cumhall was killed in the Battle of Cnucha by Goll mac Morna who became the new leader of the Fianna.
When Tadg learned Muirne was pregnant he rejected her and ordered his people to burn her to death. Conn did not allow this to happen. He sent Muirne away from the kingdom under the protection of a trusted ally.
Muirne’s son Demne was born in secret and raised by Cumhall’s sister, the druidess Bodhmall and her companion Liath Luachra. The two warrior women cared for Demne and kept him safe, by sending him to be apprenticed to the poet, Finn Ẻces (Finnegas) who lived deep in the woods.
One afternoon when Finnegas and his young apprentice were sitting on the banks of the River Boyne. Finnegas told Demne of his seven-year quest to capture the salmon of knowledge that lived in Fec’s Pool of the River Boyne where they were sitting.
According to the legend shared with Finnegas by an old druid, a magical hazelnut tree once grew at the edge of Fec’s Pool. The magical tree which contained the knowledge of the world was nourished by the Well of Segais. One of the salmon swimming in the River Boyne ate some nuts that fell into the water. As the nuts were digested, their knowledge was imparted to the salmon, and it became the smartest creature on Earth. According to the old druid if the salmon were caught, whoever ate the whole fish would be gifted with its knowledge and become the smartest person on Earth.
While he was talking, Finnegas saw a glint of light in the water. He dove in and captured the prized salmon. He instructed young Demne to cook the fish for him while he went to fetch something from his house. Finnegas admonished Demne not to taste even one bite of the fish.
Demne promised to obey. He built a fire and began cooking the salmon. When it was time to turn the fish over, Demne’s thumb slipped off the fish and into the fire. He quickly pulled his thumb away and without thinking put it in his mouth. When he tasted the juices from the fish, he realized his mistake, but the deed was already done.
When Finnegas returned, he sensed that something happened. After Demne confessed his mistake, Finnegas realized Demne had to be the one to eat the fish.
Demne ate the fish, but nothing happened until he placed his thumb in his mouth again. An energy surged through his body as the knowledge of the world lodged in his brain.
Demne’s hair mysteriously turned white, and Finnegas renamed him Fionn meaning white or fair haired.
Fionn grew up to become a mighty warrior and the leader of the Fianns.
Source for the original Irish tale:
Keith O’Hara (legend told to him in childhood)
Originally found in The Boyhood Deeds of Fionn a medieval Irish narrative to the Fenian Cycle of Irish mythology.
https://www.theirishroadtrip.com/irish-mythology/