The 12th century text Macgnímartha Finn (Boyhood Deeds of Fionn) recounts the boyhood exploits of Fionn mac Cumhaill.
The story begins with the death of Cumhal, leader of the Fianna, at the hands of Goll mac Morna. Cumhal’s wife Muirne was pregnant at the time and eventually gave birth to their son, Demne. Fearing for his safety, she sends him to be raised by Cumhal’s sister, the druidess Bodhmall, and her companion Liath Luachra. The two warrior women raise him and accompany him on several adventures, including one in which he receives his nickname, Fionn (the fair; the pale).
He developed great wisdom after inadvertently tasting the salmon of wisdom which granted universal knowledge to whoever consumed it. The salmon, which dwelled in the pool of Fés, was coveted seven years by Finn’s mentor, the poet Finn Éces. Finn cooked the salmon, obeying his mentor’s instruction not to partake any of the salmon before serving it to him, but burnt his thumb while cooking and sucked it, thereby receiving its gift of wisdom. (Though it is not stated, it is inferred that this was a Salmon of Wisdom that ate the hazelnuts at the Well of Segais.)
Fionn travels to the capital of ancient Ireland, Tara, which for the last 23 years had been set aflame each Samhain by Aillén the Burner, one of the Tuatha Dé Danann, often described as an evil goblin. Aillén played the harp and was known to sing beautiful songs. Aillén’s annual visitation to Tara was from an underwater, otherworld paradise realm inhabited by deities, from which few mortals were granted access, Mag Mell (Magh Meall – modern Irish), ruled over by Manannán mac Lir.
The ruler who had killed Fionn’s father, Goll mac Morna, and the Fianna are powerless to stop Aillén’s destruction since he puts everyone to sleep with a magical tune.
Fionn inhales poison from his own spear to prevent sleep, and laid in wait for Aillén to get near. As soon as the goblin was in striking distance, Fionn stabbed him with the spear, killing the goblin to the joy of many.
Fionn then reveals his true identity to the court, and the king grants Fionn his rightful position as leader of the Fianna. Goll steps down, and engages in a truce.
From that day forward, the event was celebrated with a huge Bonfire that acted as a beacon of hope atop the Hill of Tara. When the flames were seen other bonfires were lit, uniting ancient Ireland in it’s celebration of Samhain as a remembrance to its wise ruler, Fionn mac Cumhaill.