Rionach was a descendant of Salazar Slytherin, one of the founders of Hogwarts School of Witchcraft and Wizardry. She was born into the Irish branch of the Gaunt family. By 1603, she had married a pure-blood wizard, William Sayre, and was living at Ilvermorny Cottage in Coomloughra, County Kerry, Ireland. Their daughter, Isolt, was born that year.
She and her husband loved their daughter very much and were quietly helpful to their Muggle neighbours, producing magical cures for humans and livestock alike. She did not seem to share her family's belief in their pure blood superiority.
However, when Isolt was five, William and Rionach were killed in a fire made by her estranged sister, Gormlaith Gaunt, who "saved" Isolt with the intention of raising her with a belief in pure-blood superiority. Isolt eventually fled Gormlaith and moved to the New World where she befriended a Pukwudgie who she named William in honour of her father. Rionach had two granddaughters through Isolt's marriage to James Steward, Martha and Rionach Steward, as well as two grandsons by adoption, Chadwick and Webster Boot.