Fionn mac Cumhaill

Fionn mac Cumhaill transcribed in English as Finn McCool or Finn MacCoul, was a mythical 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" is actually a nickname meaning "blond", "fair", "white", or "bright". His childhood name was Deimne literally "sureness" or "certainty", and several legends tell how he gained the nickname when his hair turned prematurely white. The name "Fionn" is related to the Welsh name "Gwyn", as in the mythological figure Gwyn ap Nudd, and to the continental Celtic "Vindos", an epithet for the god Belenus.

The 19th-century Irish revolutionary organisation known as the Fenian Brotherhood took its name from these legends.

Source: Wikipedia