Princess Gráinne, daughter of the High King of Ireland Cormac mac Airt is the betrothed of Fionn mac Cumhaill, leader of the Fianna knight and lord of Diarmuid Ua Duibhne, the Lancer of the 4th Holy Grail War.

True to the legend, she fell in love with Diarmuid under the influcence of his Mystic Face (originated from the beauty mark under his right eye), and placed a geis on him so that he would run away with her on the night of her wedding to Fionn. As Diarmuid considered the geis a test of his honor, and knew that Gráinne fell prey to his curse, Diarmuid accepted her proposal and the two fled, besieged by Fionn's many warriors on the way. Finally, when Fionn accepted their marriage, Diarmuid was overjoyed to return to his service.

During a wild boar hunt, Diarmuid was fatally injured, and nothing but water drunk from Fionn's hands could save him. However, due to his lingering resentment, Fionn let the water slipped from his hands twice, and when he brought the water back to Diarmuid for the third time, Diarmuid was dead.