Fionn, who had several characteristics of Ireland, was born with the name of Deimne, from Cumhall mac Trénmhoir, head of the Fianna, and Muirne Muincháem, daughter of the druid Tadg mac Nuadat, who was against the union between the two. For this reason, Cumhall was forced to kidnap Muirne to be with her. At the time, there were many local kings in Ireland, but fighting the two lovers, Druid Tadg even turned to the High King of Ireland, Conn, who declared Cumhall an outlaw. The two clashed in the Battle of Cnucha, where the second was killed and at the head of the Fianna, a military body to protect the High King, was placed Goll mac Morna, who had murdered him.
Muirne was pregnant with Cumhall. Her father then repudiated her and ordered that she be burned, but High King Conn did not allow it, placing her under his protection with Fiacal mac Conchinn and his wife, Druid Bodhmall, as well as Muirne’s paternal aunt. Here Deimne was born, later called Fionn.
Muirne entrusted his son to the care of Bodhmall and the warrior Liath Luachra, who raised and trained him in the forest of Sliabh Bladma, so, once he grew up, he served some local kings, but these, each time, recognized him as the son of Cumhall and sent him away, frightened by his enemies. In the meantime, Deimne, still very young, met the Druid Finnegas and became a pupil of it. The druid had sought, for 7 years, the Salmon of wisdom, in the waters of the Boyne River. The Salmon of Wisdom made possible to obtain omniscience, but the two caught the Salmon of Knowledge, which Deimne cooked. Just as he roasted it, however, he burned his thumb and instinctively put it in his mouth, ending up feeling part of that salmon, thus acquiring the promised powers, which allowed him instantly to understand how to take revenge on his father’s killer. Finnegas then made him eat all the salmon and, since his hair was whitened early, he began to call him “Fionn”. In his life he was therefore able to use, as needed, the Thumb of Knowledge, also called, the Tooth of Winsdom/Knowledge.
Every 23 years, in Tara, on the occasion of the celebration of Samhain, a supernatural being, Áillen mac Midgna, of the Tuatha Dé Danaan, also called The Fire-Breather, burned the city after having made its inhabitants fall asleep with magnificent music, played with a dulcimer. The Fianna of Goll mac Morna were unable to counteract the fatal creature. Fionn (according to some sources he was not an adult, but he was 10 years old), however, proposed himself as a volunteer, equipped with the crane leather bag belonging to his father, containing magical weapons, he managed to resist the celestial music of the monster by pricking his nose and forehead with a spear (Birga), given to him by a man of his father, Fiacha mac Congha, which he then used to mortally strike the invader. At that point Fionn was recognized as the son of Cumhall and was placed at the head of the Fianna, welcomed also by Goll, who gave up his role and became a faithful ally. Strengthened by the power obtained, Fionn asked for compensation to his maternal grandfather, threatening him, he left his home and the entire hill of Alan. He also received compensation from Goll.
He had several wives, bur first Fionn married Sadhbh, a poetess. The two met while he was hunting and found her in the form of a female deer (the name Deimne was also used for young male deer), since Fear Doirich, furious and in love, had launched a spell. The dogs of Fionn, Bran and Sceólang, humans transformed into hounds, recognized her, so that their master did not kill her and she returned to being a beautiful woman, thanks to the magic of Allen’s fortress, in which the spell had no effect. Sadhbh became pregnant. During her pregnancy, Fionn was forced to leave her alone to repel some invaders, giving Faer Doirich the opportunity to deceive her and take her outside. Fionn would never find her again, but he had been able to join his son, Oisín, in the form of a fawn. He, in his human form, became a member of the Fianna, but also a poet like his mother, thus ending up poetizing the fate of his father.
Then he started to get busy. According to a legend, he was the architect of the construction of the Giant’s Causeway, which at the time would have connected Scotland and Ireland, with the Fingal’s Cave on Scottish soil, which actually has similar geological characteristics. According to another legend, he was able to launch an entire part of Ireland against his rival Scotland, but he erred a few hundred kilometers and created the island of Mann (where he is known as a giant), Rockall and the Lough Neagh. With the help of his wife Oona, he managed to escape when the giant Benandonner (or perhaps a fair warrior) went to his fortress to face him. Fionn pretended to be a child while the giant was being served iron-containing cakes, so he could not bite into them. Oona then served a true cake to Fionn’s fake son and when the enemy saw that the infant was able to bite into a cake that he had not been able to scratch, he went away frightened. According to other versions, he thought that if Fionn’s baby was as big as an adult, Fionn should also be huge.
Many years after the first marriage, High King Cormac mac Airt’s daughter, Gráinne, was promised in marriage, who, however, during the wedding celebrations, fled with another Fianna, Diarmuid Ua Duibhn, godson of the god Aengus, who he helped escape from the furious Fionn. The years passed and Fionn pretended to make up with the two, but, after the wounding of Diarmuid by a boar, Fionn faked to water him from his hands, a gesture that had the power to heal the dying, but, not doing it, the rival in love died, also causing the ire of Oscar, nephew of the leader of the Fianna.
A few is known of Fionn’s death. He could be killed by the same man who had killed his father, Goll, after their fellowship had cracked following a dispute over Slanga’s pig, who also divided their respective clans, but no sources tell this.
Rumor mill takes his dead body into a cathedral in Sweden, without a reason.
According to some versions, however, he would not have died, instead he would reside in a mysterious mine under Dublin, sleeping for centuries, ready to wake up to defend his Ireland. A later legend considers him reincarnated in Mongán in the seventh century BC. The most famous legend tells he is sleeping with all the Fianna in an Irish cave, waiting for the sound of the Dord Fiann, the hunting horn of them (maybe its the same Fanes are waiting for in the mountain on Braies lake), that will awake them from the sleep and they will come back, stronger than ever.
Finally, according to Geoffrey Keating, Historician lived in the seventeenth, and other researchers of the nineteenth century, Fionn probably really existed (but maybe he didn’t really do all those things). Heinrich Zimmer identified him as the Norse-Gael warrior Caittil Find, died in the the battle of Munster (857 AC). Many Icelandic documents report he was in Ireland before and probably also in Wales, but George Henderson proved that also the ancestors of Caittil had Norse names and origins too, so he wasn’t the historical Fionn.
For now, therefore, there seems to be no historical proof of the existence of Fionn mac Cumhaill, neither written nor oral, but perhaps, simply, this is the story of a man who, like many others, lived in an epoch in which History and legend still intertwined in the lives of human beings, in a Europe that still did not know it was it.