Hero of Tóraigheacht Dhiarmada agus Ghráinne [The Pursuit of Diarmait and Gráinne] of the Fenian Cycle, leading member of Fionn mac Cumhaill's Fianna, possessor of the ball seirce [love-spot], and one of the greatest lovers of early Irish literature. His father is usually named Donn, sometimes Corc; his paternal grandfather is Duibne. His mother is usually given as Cochrann, daughter of Cathaír Mór, and sometimes as Cróchnat. Regardless of his parentage, Diarmait is most linked with his divine fosterer and patron, Angus Óg. Several of Diarmait's weapons are known by specific names: his great spear, Gáe Derg [red spear], Crann Buí [yellow shaft], the smaller Gáe Buide [yellow spear], and a sword, Nóralltach [great fury], also attributed to Angus Óg and Manannán mac Lir.
Shortly after his birth, Diarmait's father Donn takes him to be fostered by Angus Óg at Brug na Bóinne. While there, Donn learns that his wife has lain in adultery with Roc, Angus Óg's steward. She gives birth to a child, whom Donn murders but whom his father, Roc, restores to life as a great boar with a magical wand. Roc orders the boar to hunt Diarmait to death, which he accomplishes in Tóraigheacht Dhiarmada agus Ghráinne. In most versions the boar roams near Ben Bulben, Co. Sligo, but other areas in both Ireland and Scotland have claimed him as well.
Another well-known story explains his ball seirce, which made him irresistible to women. Already the handsomest of the Fianna, Diarmait is hunting with Goll, Conán mac Morna, and Oscar when he comes upon a beautiful young girl living with an old man, his cat, and his wether (gelded sheep) in a hut in the forest. As they all sit down to eat, the old man's cat jumps up on the table, but none of the Fianna can take it off. The old man explains that the wether is the world and the cat death. After the four men have gone to bed, the beautiful young girl sleeps in the same room with them. Each of the men desires to make love with her, but she rejects all but Diarmait, declaring that she is Youth incarnate and that she will put a spot on her lover that no woman may see without loving him. Touching his forehead, she gives him the ball seirce.
The hero's best-known story is told in Tóraigheacht Dhiarmada agus Ghráinne, in which Diarmait elopes with young Gráinne while she is betrothed to the aged Fionn. The lovers are pursued all over Ireland for many years, until Diarmait is gored by a boar and allowed to die by Fionn. Diarmait also participates in the chase for Ábartach [the Hard Gilly] and has adventures in the Otherworld. His counterpart in the Ulster Cycle is Noíse, lover of Deirdre. He is also compared with Adonis of classical mythology and Osiris of Egyptian. The shadowy Ferdoman of the Fenian Cycle may anticipate him.