Fionn mac Cumhaill is a celebrated hero in Irish literature, with stories concerning him continuous in the literature for well over a thousand years. Fionn is always portrayed as a great warrior and seer. As a seven-year-old boy, he met on the banks of the Boyne with a seer called Finneigeas. Finneigeas had dedicated the past seven years of his life to catching the Salmon of Knowledge, which swam in the river and would impart the knowledge of the world on the first person to taste it .

While Fionn was there, Finneigeas caught the salmon, and with much joy put it on the spit to cook, entrusting the cooking to Fionn but warning him not to taste it. After a time, Fionn went to see if the fish was cooked, however, he touched it with his thumb and burnt himself, leaving a blister. To ease the pain, he put his thumb in his mouth, and thus became the first person to taste the salmon. When Finneigeas looked at the boy's face, he saw the wisdom shining in it, and knew that the salmon was no good to him. Ever after that, if Fionn needed to know something, he put his thumb into his mouth and the knowledge came to him.

When Fionn was ten years old, he went to the court of Conn Ceadchathach at Tara for the November feast. The king was desolate, for every other year on that night the citadel was burned down by an otherworld being called Aillen, who lulled everyone to sleep with his magical music. Fionn offered to stand guard, and was given a spear by Fiacha. When he heard the magical harp-music, he pressed the point of the spear into his forehead, and the pain kept him awake. He jumped up to face Aillen, who released a blaze of fire from his mouth, but Fionn quenched the blaze with his cloak and put Aillen to flight. Chasing him to Sliabh Fuaidh in Co. Armagh, Fionn cast his spear at Aillen and slew him. He then returned to Tara, and Conn Ceadchathach was so grateful that he installed Fionn as leader of all the Fianna in Ireland.

Fionn is portrayed as a great hunter of deer and wild pigs, and had many hounds, including two especially great hounds, Bran and Sceolaing. The king of the Dal nAraidhe, in Co.Antrim and Co.Down, desired Fionn's aunt Uirne as a wife, and Fionn agreed to the marriage. Howeve, the king's first wife was jealous and turned Uirne into a hound. The warrior Luaghaidh Lagha slew the king as a result, and Uirne regained her shape and married Luaghaidh. She bore him triplets, but at the same time brought forth two pups. They were the cousins of Fionn, and became Bran and Sceolaing, his two hunting dogs.

Bran was Fionn's favourite, and he loved it intensely. Bran and Fionn made great noise together at feasts, and whenever any of the Fianna were hungry Bran would go into the forest and bring their meal to them. However, once Bran was yelping impatiently, and in anger Fionn struck it on the head with his whip. Bran stared at its master with tear-filled eyes, then wrenched itself free and raced to a lake where it drowned itself. Every time after that when Fionn heard the baying of a hunting hound, his heart nearly broke.