The Pursuit of Diarmuid and Gráinne (Irish: Tóraigheacht Dhiarmada agus Ghráinne or Tóraíocht Dhiarmada agus Ghráinne in modern spelling) is an Irish prose narrative surviving in many variants. A tale from the Fenian Cycle of Irish mythology, it concerns a love triangle between the great warrior Fionn mac Cumhaill, the beautiful princess Gráinne, and her paramour Diarmuid Ua Duibhne. Surviving texts are all in Modern Irish and the earliest dates to the 16th century, but some elements of the material date as far back as the 10th century.
The story begins with the ageing Fionn, leader of the warrior band the Fianna, grieving over the death of his wife Maigneis. His men find that Gráinne, the daughter of High King Cormac mac Airt, is the worthiest of all women and arrangements are made for their wedding. At their betrothal feast, however, Gráinne is distressed that Fionn is older than her father, and becomes enamored with Fionn's handsome warrior Diarmuid (according to oral versions, this is because of the magical "love spot" on his forehead that makes him irresistible.) She slips a sleeping potion to the rest of the guests and encourages Diarmuid to run away with her. He refuses at first out of loyalty to Fionn, but relents when she threatens him with a geis forcing him to comply. They hide in a forest across the River Shannon, and Fionn immediately pursues them. They evade him several times with the help of other Fianna members and Aengus Óg, Diarmuid's foster father, who conceals Gráinne in his cloak of invisibility while Diarmuid leaps over the pursuers' heads.
Different variants from Ireland and Scotland contain different episodes, sending Diarmuid and Gráinne to all manner of places. Commonly Diarmuid refuses to sleep with Gráinne at first out of respect for Fionn; in one version she teases that water that has splashed up her leg is more adventurous than he is. A similar quip appears in some versions of the Tristan and Iseult legend. Another episode describes how the newly-pregnant Gráinne develops a craving for rowan berries guarded by the one eyed giant Searbhán; though at first friendly to the lovers, Searbhán angrily refuses to give up the berries and Diarmuid must fight him. Searbhán's skill at magic protects him from Diarmuid's mortal weapons, but Diarmuid eventually triumphs by turning the giant's iron club against him.