Fionn gained command of the Fianna by saving the life of the High King Cormac mac Airt, who later promised his daughter Gráinne (grahn-ya) to him in gratitude for a lifetime of service and honor. 

    As a relatively old man, Fionn sees Gráinne and falls in love with her. Cormac promises that she will be wed to Fionn. Diarmaid Ua Duibhne (deer-mwid a div-na) is the son of one of Fionn's original band of Fianna and one of his most trusted and beloved friends. He is also the most handsome of all the Fianna. Gráinne does not want to marry Fionn, and so she selects Diarmaid to help her to avoid fulfilling her father's promise. On the night before the wedding, Gráinne puts a sleeping potion into the great cask of wine to be drank at that night's feast. She bids Diarmaid not to drink even a drop of wine that night, and promises to tell him why later on. After everyone in the place has passed out from the drugged wine, the two ride away for a few stolen moments together. Gráinne convinces Diarmaid to take her away from Tara and be her lover, saying she could not bear for the rest of her life to be married to Fionn, who was old and whom she did not love, when her true love was in her arms at that very moment. Against his better judgement, Diarmaid runs away with Gráinne. For the next sixteen years, Fionn and his band of Fianna pursue the lovers, until eventually he is persuaded to put aside his anger and make peace with Diarmaid. Diarmaid and Gráinne set up house very near Fionn, and had four sons and a daughter. Fionn, Diarmaid and a small entourage of Fianna went out hunting one day, and chased a great boar deep into the forest near the great mountain of BenBulben in Sligo. Diarmaid and Fionn separated from the group in pursuit of the powerful animal. Diarmaid is gored by the beast and dying, asks Fionn for a last drink of water. Fionn goes to the stream to fetch the water, only to realize that he has no vessel in which to carry it back. He cups his hands and carries the water back to Diarmaid, but when he reaches his dying friend, he is overcome with grief to find that the water has all run through his fingers and his hands are now empty. Diarmaid dies with his last dying request unfulfilled, and Fionn is haunted by it for the rest of his life. He would always feel that he had failed Diarmaid.