Fionn mac Cumhaill, anglicised to Finn McCool was a mythical giant, a hunter-warrior of Irish mythology, occurring also in the mythologies of Scotland and the Isle of Man. The stories of Fionn and his followers, the Fianna, form the Fenian cycle or Fiannaidheacht, much of it purported to be narrated by Fionn's son, the poet Oisín.

Fionn or Finn is actually a nickname meaning "fair" (in reference to hair and/or skin colour), "white", or "bright" and several legends tell how he gained the nickname when his hair turned prematurely white. Most of Fionn's early adventures are recounted in the narrative The Boyhood Deeds of Fionn. He was the son of Cumhall – leader of the Fianna – and Muirne, daughter of the druid Tadg mac Nuadat who lived on the hill of Almu in County Kildare. Cumhall abducted Muirne after her father refused him her hand, so Tadg appealed to the High King, Conn of the Hundred Battles, who outlawed him. The Battle of Cnucha was fought between Conn and Cumhall, and Cumhall was killed by Goll mac Morna, who took over leadership of the Fianna. Muirne was already pregnant, so her father rejected her and ordered his people to burn her, but Conn would not allow it and put her under the protection of Fiacal mac Conchinn, whose wife, Bodhmall the druidess, was Cumhall's sister. In Fiacal's house she gave birth to a son, whom she called Deimne.

Muirne left the boy in the care of Bodhmall and a warrior woman, Liath Luachra, who brought him up in secret in the forest of Sliabh Bladma, teaching him the arts of war and hunting. As he grew older he entered the service – incognito – of a number of local kings, but when they recognised him as Cumhal's son, they told him to leave, fearing they would be unable to protect him from his enemies.

The young Fionn met the leprechaun-like druid and poet Finn Eces, or Finnegas, near the river Boyne and studied under him. Finneces had spent seven years trying to catch the salmon of knowledge, which lived in a pool on the Boyne: whoever ate the salmon would gain all the knowledge in the world. Eventually he caught it, and told the boy to cook it for him. While cooking it Fionn burned his thumb, and instinctively put his thumb in his mouth, swallowing a piece of the salmon's skin. This imbued him with the salmon's wisdom. He then knew how to gain revenge against Goll, and in subsequent stories was able to call on the knowledge of the salmon by sucking his thumb. The story of Fionn and the salmon of knowledge bears a strong resemblance to the Welsh tale of Gwion Bach. It is believed that the descendants of this Salmon swam north to the O'Neill county of Tyrone and settled there. Legend has it that Beraghs Seamus Boyle consumed the juicy flesh of one of these fish at a social gathering, and aquired unimaginable knowledge and wisdom

Fionn met his most famous wife, Sadhbh, when he was out hunting. She had been turned into a deer by a druid, Fear Doirich, for she had refused to marry him. Fionn's hounds, Bran and Sceolan, who were once human themselves, recognised she was human, and Fionn spared her. She transformed back into a beautiful woman the moment she set foot on Fionn's land, as this was the one place she could regain her true form. She and Fionn married and she was soon pregnant. However, Fear Doirich (literally meaning Dark Man) returned and turned her back into a deer, whereupon she vanished. Fionn spent seven years searching for her, but to no avail. Fortunately, he was later reunited with their son, Oisín, who went on to be one of the greatest of the Fianna.

One day when going about his daily business a Scottish Giant named Fingal began to shout insults and hurl abuse at him from across the channel. In anger Finn lifted a clod of earth and threw it at the giant as a challenge, the earth landed in the sea. Fingal retaliated with a rock thrown back at Finn and shouted that Finn was lucky that he wasn't a strong swimmer or he would have made sure he could never fight again. Finn was enraged and began lifting huge clumps of earth from the shore, throwing them so as to make a pathway for the Scottish giant to come and face him.

However by the time he finished making the crossing he had not slept for a week and so instead devised a cunning plan to fool the Scot. Finn diguised himself as a baby in a cot and when his adversary came to face him Finn's wife told the Giant that Finn was away but showed him his son sleeping in the cradle. The Scottish giant became apprehensive, for if the son was so huge, what size would the father be? In his haste to escape Fingal sped back along the causeway Finn had built, tearing it up as he went. One clod of earth was left behind and this became the Isle of Man. The hole that the clod came from filled with water and became Lough Neagh. He is said to have fled to a cave on Staffa which is to this day named 'Fingal's Cave'

In The Pursuit of Diarmuid and Gráinne – one of the most famous stories of the cycle – the High King Cormac mac Airt promises the now aging Fionn his daughter Gráinne as his bride, but Gráinne falls instead for one of the Fianna, Diarmuid Ua Duibhne, and the pair runs away together with Fionn in pursuit. The lovers are aided by Diarmuid's foster-father, the god Aengus. Eventually Fionn makes his peace with the couple. Years later, however, Fionn invites Diarmuid on a boar hunt, and Diarmuid is badly gored by their quarry. Water drunk from Fionn's hands has the power of healing, but when Fionn gathers water he deliberately lets it run through his fingers before he gets back to Diarmuid. His grandson Oscar threatens him if does not bring water for Diarmuid, but when Fionn finally returns it is too late; Diarmuid has died.

Accounts of Fionn's death vary; according to the most popular, he is not dead at all, rather, he sleeps in a cave below Dublin, surrounded by the rest of the Fianna. One day they will awake and defend Ireland in the hour of her greatest need. In one account, it is said they will arise when the Dord Fiann (his hunting horn) is sounded three times, and they will be as strong and as well as they ever were.

The 19th century Irish revolutionary organisation known as the Fenian Brotherhood took its name from these legends. The Scottish name Fingal comes from a retelling of these legends in epic form by the 18th century poet James Macpherson.