Irish bard warrior of legend from the Fenian Cycle of Irish mythology. He's the son of Finn MacCool/Fionn mac Cumhaill.
He marries a faery woman and goes off with her to Tir na nÓg, the faery land, and becomes its king. However, after fathering a son and daughter, he decides to return, but 300 years have passed in our world. He messes up instructions from his wife, and ends up aging 300 years instantly. He manages to hang on long enough to pass on all his tales and poems to good auld St. Patrick before he dies.
His name means young deer or faun.