https://en.wikipedia.org/wiki/Gormflaith_ingen_Murchada

Gormflaith ingen Murchada [pronunciation?] was born in Naas, County Kildare, Ireland, around 960. She was the daughter of Murchad mac Finn, King of Leinster, sister of his successor, Mael Mórdha mac Murchada. According to the annalistic accounts, she was married to Olaf Cuaran, the Viking king of Dublin and York until his death in 981;[1] and mother to his son, King Sigtrygg Silkbeard.[2] Additionally, and perhaps most famously, after her son, Sigtrygg's defeat at the Battle of Glen Mama in 999, Gormlaith was married to Brian Boru, the King of Munster and High King of Ireland, and mother to his son and later King of Munster, Donnchad. It is also alleged that she married, Máel Sechnaill mac Domnaill after Olaf's death, but this is somewhat contentious as the sources for this marriage are less reliable. The Irish annals record Gormlaith's death in 1030. Gormlaith is most infamous for allegedly inciting men to such a degree that she caused the Battle of Clontarf in 1014. Given that her goading episode exists only in literary sources not contemporary with her lifetime, and, indeed, written long after her death, it is highly unlikely that such an event ever occurred.[3] Since the majority of depictions of Gormlaith were composed well after her life, very little is known about the actual historical figure.

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http://en.wikipedia.org/wiki/Gormflaith

Gormflaith was born in Naas, County Kildare, Ireland, around AD 960. She was the daughter of Murchad mac Find, King of Leinster, sister of his successor, Mael Mórdha mac Murchada, and widow of Olaf Cuaran, the Viking king of Dublin and York. The main source of her life history is the Cogadh Gaedhil re Gallaibh. She was also the mother of King Sigtrygg Silkbeard of Dublin.
Gormflaith married Máel Sechnaill mac Domnaill after Olaf's death, but she 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 Njál's saga, which refers to her as "Kormloð": "she was endowed with great beauty... [but] was utterly wicked." She was later divorced by Brian, and she began engineering opposition 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. Brian's forces were victorious, however, and neither Gormflaith nor Sigtrygg were killed, as they were safe behind the walls of Dublin. She died in 1030.