The flight of Diarmuid and Grainne began at the wedding between ageing Fionn, leader of the Fianna, and Gráinne, daughter of High King Cormac mac Airt. Instead of marrying Fionn, Grainne became enamoured with Fionn's handsome warrior, Diarmuid, and persuaded him to run away with her. 

They hid in a forest across from the River Shannon while Fionn pursued them, evading him with help from other Fianna members and Aengus Óg, Diarmuid's foster father. With Gráinne concealed in Aengus's cloak of invisibility, Diarmuid leaped over the pursuers’ heads. 

The couple fled from their pursuers and their legendary flight together is well established in mythical lore.

After many adventures, including newly-pregnant Gráinne craving rowan berries guarded by one-eyed giant Searbhán who angrily refused to let them eat so Diarmuid fought against him. Searbhán's magic initially protected him from Diarmuid's mortal weapons but Diarmuid eventually triumphed by turning the giant's iron club against him.

Aengus negotiated peace with Fionn and the lovers settled in Keshcorran, County Sligo where they had five children. 

In time Fionn organised a boar hunt near Benbulbin and Diarmuid joined the hunt, despite knowledge of a prediction he would be killed by a boar. The creature wounded him mortally just as he struck it a fatal blow. 

By letting him drink water from his hands, Fionn had the power to heal his dying comrade but he let the water slip through his fingers twice. Finally Fionn's grandson Oscar threatened him with violence if he did not help Diarmuid, but when Fionn returned from the well on the third attempt it was too late. Diarmuid was dead.

Aengus took Diarmuid's body to his home at Brú na Bóinne. Whether Gráinne swore her children to avenge their father's death upon Fionn, grieving until she herself died; or was reconciled with Fionn, negotiating peace between him and her sons; or even went so far as to marry Fionn at last, is left to artists and storytellers of myths and legends to decipher and depict.