Diarmuid Ua Duibhne or Diarmid O'Dyna (also known as Diarmuid of the love spot) is a son of Donn and a warrior of the Fianna in the Fenian Cycle of Irish mythology. He is most famous as the lover of Gráinne, the intended wife of Fianna leader Fionn mac Cumhaill in The Pursuit of Diarmuid and Gráinne. Aengus Óg is Diarmuid's foster-father and protector.

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.

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A geis (or geasa) can be compared with a curse or, paradoxically, a gift. If someone under a geis violates the associated taboo, the infractor will suffer dishonour or even death. On the other hand, the observing of one's geasa is believed to bring power and good fortune. Often it is women who place geasa upon men. In some cases the woman turns out to be goddess or other sovereignty figure.

The geis is often a key device in hero tales, such as that of Cúchulainn in Irish mythology. Traditionally, the doom of the hero comes about due to their violation of their geis, either by accident, or by having multiple geasa and then being placed in a position where they have no option but to violate one geis in order to maintain another. For instance, Cúchulainn has a geis to never eat dog meat, and he is also bound by a geis to eat any food offered to him by a woman. When a hag offers him dog meat, he has no way to emerge from the situation unscathed; this leads to his death.