Gormflaith was born in Naas, County Kildare, Ireland, around AD 960. She was the daughter of Murchad MacFinn, King of Leinster, sister of his sucessor, Mael Mordha, and widow of Olaf Cuaran, the Viking king of Dublin and York. The main source of her life is the Cogadh Gaedhil re Gallaibh. She was also the mother of King Sigtrygg Silkbeard of Dublin.

Gormflaith is best known for being the third wife of Brian Ború, the High King of Ireland. She was the mother of Donnchad, who succeeded Brian as King of Munster. In 999, Brian defeated Mael Mordha and Sigtrygg 'Silkbeard' at the Battle of Glen Mama.. To negotiate peace, Brian married one of his daughters to Sigtrygg and took Gormflaith as wife. According to Njal's Saga, which refers to her as "Kormlod": "she was endowed with great beauty... [but] was utterly wicked." She was later divorced by Brian, and she began engineering oppostition to the High King. She prompted Sigtrygg into gathering support from Vikings outside Ireland, most notably Earl Sigurd of Orkney and Brodir of the Isle of Man.

The conflict she caused came to its climax at the Battle of Clontarf, at which Brian was killed. The Irishmen were victorious, however, but neither Gormflaith nor Sigtrygg were killed, as they were safe behind the walls of Dublin. She died in 1030.