Diarmuid Ua Duibhne

Diarmuid Ua Duibhne appears in the Fenian Cycle as a member of the Fianna who is fostered by Aengus Óg of the Tuatha Dé Danann and raised at Brú na Bóinne. His lineage is signalled by his patronymic (Ua Duibhne = 'grandson/descendant of Duibne'). Narrative episodes record fosterage under Aengus Óg, his rise as a prominent warrior of the Fianna, and conflicts that lead to fatal geasa and druidic reprisals—most notably an episode in which a grieving father, aided by a druid, resurrects his dead son as a boar and binds the creature as part of an act meant to bring about Diarmuid's death.

Primary descriptive markers in the tradition focus on Diarmuid's attractiveness and a specific magical mark: the ball seirce (a 'love spot') bestowed by a personification of Youth. The ball seirce is said to make him irresistible to any woman who sees him, and he is often referred to in sources as 'Diarmuid of the Love Spot.' Extant summaries emphasize his reputation as handsome and popular with women but provide few consistent details on other aspects of his physical appearance.

Diarmuid is portrayed as an exceptional warrior—sources recount an episode in which he saves Fionn and the Fianna by single-handedly slaying over three thousand enemies—and as a liminal figure supported by otherworldly patronage (foster-son of Aengus Óg). His adventures include receiving the ball seirce, hospitable acceptance of an ugly visitor that leads to an otherworldly household, an enchanted voyage 'out over the sea, and then down below it' to the Land-under-Waves where he collects three drops of blood and seeks three draughts from the cup of the King of Magh an Ionganaidh to cure an enchanted woman. He also figures centrally in the romantic-tragic narrative in which Gráinne elopes with him, precipitating the Pursuit of Diarmuid and Gráinne.