When Fionn was only a few months old his mother Muirne took him into the woods of Sliabh Bladhma to be cared for by two women, Bodhmall the Druidess and Liath Luachra, who was a warrior who trained other warriors. His father Cumhail had been killed by the sons of Morna and it was unsafe for him to remain with his mother.
Bodhmall and Liath taught him wisdom and strength; the ways of the druid and the ways of the warrior. They set him many tests of his talents. They taught him to run and found him fleet of foot, they taught him the art of swimming and found he was at home in the water. He was taken into a field with hares and was told to let none escape, which task he performed perfectly. Therefore the women declared him a worthy champion though as yet he had fought no battle except in training with Liath.
Then came the time when he was a young man and he had to leave Sliabh Bladhma as the sons of Morna were seeking to kill him. Fionn decided to enlist the help of his father's brother, his uncle Crimhall who lived in Connaught although he was not sure of the exact location.
On the way he met up with some youths who were swimming and he joined them and out swam them all in contest which did not please them very much. However they saw how fair he was in body and decided to call him Fionn which means fair or bright, so that was his name from that time onwards for he had no name of his own before then.
Then Fionn went to Carraighe and was employed by a king who unknown to him was married to his mother. One night as they were playing chess and Fionn had won seven games in a row, the king looked up and eyed him narrowly and said 'You are the son of Cumhail, that was lord of the Fianna. I bid you to leave, for I do not wish it to be said that you were killed while under my protection.'
Fionn travelled on searching all the while for Crimhall. Then he met a woman who was crying tears of blood for the loss of her son at the hands of a great warrior who lived nearby. Fionn said he would avenge the death for the woman and went up against the warrior and slew him, thus fighting his first battle and it turned out that the warrior he had slain had been the first to strike a blow at his father and had carried off his father's crane skin treasure bag, so Fionn recovered his father's bag and with it the magical items and wisdom that was carried within.
Now that Fionn had the magical bag he could divine by its aid the location of his uncle Crimhall who was living with the last of the old Fianna who refused to serve under the sons of Morna. They were very glad to see Fionn when he arrived at their hiding place and said that they would follow his leadership only.
Fionn had a thirst for knowledge now having used the power of the crane skin bag and so he went into the service of a poet and seer name Finegas under the assumed name of Deimne and for seven years he stayed with Finegas learning all he had to teach.
Now Finegas lived by the side of the river Boinne ( now called Boyne) in the hopes that he might catch the fabled Salmon of Knowledge for there was a prophecy that he would be the one to catch it. Then one day when Fionn had served with him five years he at last caught the magical fish. He gave it to Fionn to cook, warning him not to eat of its flesh. While Fionn was cooking the fish and turning it on the pan he burnt his thumb and immediately put it in his mouth to cool the pain. When he brought the fish to Finegas he was asked 'Did you eat of the flesh?' Then Fionn remembered how he had burnt his thumb and had to explain this to Finegas. 'Your name is not Deimne, it is Fionn and the prophecy spoke of you eating the fish that I Finegas would catch' and then Finegas offered the whole fish to Fionn to eat which he did gaining the knowledge of the nine hazels of wisdom from beside the Well of Wonder which lies beneath the sea. From then on he had only to put his thumb in his mouth to know whatever he wished and he became known for his wisdom throughout Ireland.
When the seven years of training with Finegas were up Fionn decided that he was ready to become a warrior and he went to the High King Cormac Mac Art at his hall in Tara, Co. Meath and announced that he was Fionn son of Cumhail and that he had come to take his place among the Fianna and to serve Cormac. Cormac took Fionn into the Fianna although Goll Mac Morna and his brothers murmured against this particularly Goll who was now the captain of the Fianna having helped to kill Cumhail, Fionn's father.
It was nearing Samhain and every year for the past nine years a warrior from the Sidhe Finnachaidh of the Tuatha De Danann by the name of Aillen Mac Midhna came to Tara to cause havoc among the Fianna. He had burned the roof of Tara with his magic and had caused all the warriors to fall into a deep sleep with his Faery music.
When he heard this Fionn went before Cormac Mac Art and promised to rid him of this nuisance providing that his right of inheritance to the title of captain be honoured. Cormac swore to fulfill this request on the surety of all the tributary kings of Ireland and all his royal Druids.
The night before the warrior of the sidhe was going to appear, one of Cormac's men Fiacha Mac Conga, who had served with Fionn's father and was therefore protective towards him, came to Fionn and offered his help. He gave Fionn a magical spear which made the sound of battle when it was unsheathed and when it was laid on the forehead of the warrior who carried it he would be protected from evil magic.
So Fionn took the magical spear and went out against Aillen Mac Midhna and killed him. He struck off his head and carried it back to Tara and put it up on a pole for all to see.
When dawn broke and the High King and all his retinue awoke from their enchanted sleep, Cormac called Fionn before him and invested him with the captaincy of the Fianna in accordance with his promise. Then he said to Goll Mac Morna 'Will you stay and serve under Fionn or would you rather be banished from this island forever?' and Goll who only had one eye, looked at Fionn and bowed his head and declared that he would serve Fionn and if he ever betrayed him that he should be put to death immediately. Goll kept his word and did not betray Fionn and from that day forward Fionn had the captaincy of the Fianna the protectors of Ireland.