|Age|| 17-21 (Daughter of the Forest)
30 (at time of death)
|Siblings||Liam, Cormack, Conor, Finbar, Padriac, Sorcha, Ciarán|
|Other relations|| Sean, Fernando, Luis, Juan (nephews)
Niamh, Liadan, Aisha, Carmen, Dolores (nieces)
Johnny, Fintan, Cormack, Coll, Finbar (great-nephews)
Muirrin, Deirdre, Clodagh, Maeve, Sibeal, Eilis (great-nieces)
Oisin, Ronan, unnamed boy (great-great-nephews)
Emer, Firinne, unnamed girl (great-great-nieces)
|Residence||Sevenwaters (until death)|
Diarmid falls in love with his stepmother, the villain sorceress Lady Oonagh, possibly under her influence.
During the curse, Diarmid feels bitter because of his role in what occurred, and wants revenge on the Lady Oonagh for doing this to him and his brothers. After the curse, Diarmid is disappointed to find that Oonagh has fled Sevenwaters, and later sets out to seek vengeance upon her. He is found by Cormack, and later his father Lord Colum, and the three of them undertake a quest to free Colum's baby son Ciaran from Oonagh's clutches. They succeed, with Conri's help, and return Ciaran to the nemetons to raise him there.
Nine years after the curse is broken, during a daring battle for the Islands, Cormack is killed by a sword through the heart, and Diarmid goes to avenge him. He is captured by the Britons and his body is found later in the sea. His body and Cormack's are brought back to Sevenwaters to be buried there. Sorcha is not allowed to see the bodies because it would hurt too much. It is implied that the Britons drowned Diarmid for sport.
Diarmid is a passionate character, typical of an "act first ask questions later" mentality. His loyalty to his family is shaken at times, but he ultimately dies avenging his brother Cormack.