Diarmuid met a woman who caused a magical love spot to appear on his head; any woman that looked at his head fell instantly in love with him. This woman was soon to be wed.
Gráinne, intended bride of Fionn mac Cumhail, fell in love with Diarmuid when she saw him in the wedding party. She laid a geis upon him to run off with her, with their long flight from Fionn aided by Aengus Óg.
Eventually, Fionn pardons Diarmuid after Aengus Og intercedes on their behalf; the pair settle in Kerry and produce five children. Years later, however, Fionn invited Diarmuid on a boar hunt, and Diarmuid was badly gored by a giant boar on the heath of Benn Gulbain. Water drunk from Fionn's hands had the power of healing, but when Fionn gathered water he would deliberately let it run through his fingers before he could bring it to Diarmuid. He had to be threatened by his son Oisín and grandson Oscar to play fair, but too late: Diarmuid had died.
The story of Diarmuid and Gráinne is one of a number of instances in Irish mythology of the eternal triangle of young man, young girl and aging suitor. This theme is seen in the tale of Naoise, Deirdre, and Conchobar.
Diarmuid Ua Duibhne is said to be the founder of the Scottish clan Campbell. On the Campbell crest is a boarshead, a nod to how Diarmuid died.