Diarmuid Ua Duibhne, also known as Diarmuid of the Love Spot, is a hero and demigod in the Fenian Cycle of Irish mythology, traditionally thought to be set in the 2nd to 4th century. He is the son of Donn, son of Duibhne of the Fianna, and Cochrann, daughter of Cathaír Mór. Diarmuid becomes the foster son and protégé of Aengus Óg, one of the Tuatha Dé Danann and the god of love, to the extent that a god of love can be said to exist in the corpus. He grows up to be a skilled warrior and a well-liked, valued member of the Fianna who on one occasion saves Fionn and his band by single-handedly slaying over three thousand enemies in battle.

The Fenian Cycle, Fianna Cycle or Finn Cycle is a body of early Irish literature focusing on the exploits of the mythical hero Finn or Fionn mac Cumhaill and his warrior band the Fianna. Sometimes called the Ossianic Cycle after its narrator Oisín, it is one of the four groupings of Irish mythology along with the Mythological Cycle, the Ulster Cycle, and the Kings' Cycles. Timewise, the Fenian cycle is the third, between the Ulster and Kings' cycles. The cycle also contains stories about other famous Fianna members, including Diarmuid, Caílte, Oisín's son Oscar, and Fionn's rival Goll mac Morna.