So who was Finn MacCool? In a battle for the position of Chieftain of the Fianna, Finn's father,Cumhail is wounded and his attacker carries off his pouch of magical objects. Lacking his pouch, Cumhail is slain by a member of the Morna clan, who beheads him. Slain, Cumhail leaves behind a pregnant wife, Muirne, who gives birth to a beautiful fair-haired boy she named Demne. Fearing for her son's life at the hands of Clan Morna, she sends him to the forest to be raised by Bodhmal the Druidess and her sister, the warrior Liath Luachra. Reared by these strong, wise women and tutored by the Druid Finegas, Demne grew to become a fierce warrior skilled at weaponry and fighting as well as at the healing and magical arts. To keep him a secret from the clan Morna, he becomes known as "Fionn", meaning "fair or fair-haired".

While Finn is growing up, the druid Finegas catches the Salmon of Knowledge and gives it to him to cook. Finn burns himself while doing so and sucks his thumb, thus acquiring the gift of prophecy, which he uses to ensure his survival, bring peace to his homeland, and inspire the Fianna to greatness. He later gains command of the Fianna by saving the life of the High King Cormac mac Airt. He has many adventures involving hunting, fighting, sorcery, love, and passion. One of his many romances is with the goddess Sadb who gives him his famous son, Oisín (Ossian).

Finn is of course best known as the creator of the Giant's Causeway. One day, Finn grows angry when he hears that a Scottish giant is mocking his fighting ability. He throws a rock across the Irish Sea to Scotland; the rock is a challenge to the giant.

The Scottish giant quickly throws a message in a rock back to Finn, stating he can't take up the challenge because he can't swim to reach Ireland. Finn doesn't let the Scottish giant off so easily. He tears down great pieces of volcanic rock that lie near the coast and stands the pieces upright, making them into pillars to form the causeway that sretches from Ireland to Scotland. The giant now has to accept the challenge and he comes to Finn's house. Finn, masquerading as a 18-foot baby, bites the Scottish giant's hand and then chases him back to Scotland, flinging huge lumps of earth after him. One of the large holes he creates fills with water and becomes Lough Neagh, the largest lake in Ireland. One large lump of earth misses the giant and falls into the Irish Sea; this lump is now known as the Isle of Man.

It is not known how Finn died or indeed if he is dead at all. Legend states that he is in fact sleeping in a cave, waiting to awaken and defend Ireland in her greatest hour of need.