Daughter of Cormac mac Airt, she was the much-younger wife of Fionn mac Cumhill. Unfortunately, she saw the "lovespot" of Diarmud, the nephew of Fionn, and instantly fell in love. Diarmud pled with her not to, but she pursued him, and he in turn fell in love with her. They fled Fionn's wrath, hiding in the woods, until Diarmud died by Fionn's hand. Grainne agreed to go then with Fionn, and stayed with him until her death.
The figure of Grainne is interesting in that it has analogies in the story of Tristan and Iseult, and in story of Deirdu (aka "The Fate of the Sons of Usnech"). Given that her name means "sun," she may also be the remnant of a solar deity, and the story of Diarmud and Fionn's battle for her is--as has been suggested by many scholars--a seasonal myth, like that of Gwyn ap Nudd and Gwythyr for Creiddylad in "Culhwch ac Olwen" and, of course, the story of Lancelot and Guenevere.