Grainne is a part of the triple goddess formed by Herself and Her two sisters, Fenne and Aine. Both Grainne and Aine were seen by locals as beautiful, golden-haired goddesses who visited their fields and hilltops to protect and nurture the land, people and animals.
A Sun Goddess and master herbalist, Grainne rules herbs, knowledge, the sun, and fire.
A large barrow on the Hill Of Tara, Rath Grainne is named after Cormac's daughter Gráinne, who was promised in marriage to Fionn but, repulsed by his age, she forms a relationship with Diarmuid at their betrothal party. At first he refuses out of loyalty to Fionn but she places a geis upon him to run away with her. Their long flight from Fionn is aided by Diarmuid's foster-father Aengus Óg. Eventually, Fionn pardons Diarmuid after Aengus intercedes on their behalf; the pair settle in Kerry and produce five children. Years later Diarmuid is wounded by a boar while hunting with Fionn, who stalls in healing him until it is too late; texts vary on Gráinne's subsequent actions. In some versions, she mourns her husband until she dies herself; in others, she swears her sons to avenge their father's death on Fionn. In still others, she forgives Fionn or even marries him.