Iseult was born on 6 August 1894, the daughter of Maud and her then married French Boulangist lover Lucien Millevoye. Maud Gonne claimed that Iseult was conceived in the mausoleum of Iseult's late brother, Georges Silvère (1890–1891) who died of meningitis, in an attempt by her parents to reincarnate their dead and still adored infant. Iseult was educated at a Carmelite convent in Laval, France; when she returned to Ireland she was referred to as Maud's niece or cousin rather than daughter.
In 1903 Maud Gonne married John MacBride; Iseult's half-brother Seaghan Sean MacBride was born in 1904 and the couple separated in 1905. The family lived in France most of the time until John MacBride's death in the 1916 Rising, with Gonne fearing that Sean's father would seize him from her. In a separation settlement, MacBride was granted a month's summer custody, however, he returned to Ireland and never saw his child again. Iseult's relationship with her stepfather was tainted by an allegation by William Butler Yeats, who wrote to Lady Gregory in January 1905 (the month MacBride and Maud separated) that he had been told MacBride had molested Iseult, who at that time was ten years old. However, many critics have suggested that Yeats may have fabricated the event due to his hatred of MacBride over Maud’s rejection of him in favour of MacBride. The divorce papers submitted by Gonne made no mention of any such incident – the only charge against MacBride that was substantiated in court was that he was drunk on one occasion during the marriage and Iseult's own writings make no mention of the allegation.
In 1913, Iseult met Rabindranath Tagore. Inspired by his poetry, she began to learn Bengali in 1914, tutored by Devabrata Mukerjea. Together, in France, they translated some of Tagore's The Gardener into French directly from the Bengali. Tagore left it to Yeats' discretion to decide the merit of the work, but Yeats did not feel sufficiently fluent in French to judge them. The translations were never published. Iseult was widely considered a great beauty, and temperate, able to speak her mind. She attracted the admiration of literary figures including Ezra Pound, Lennox Robinson and Liam O'Flaherty. Her most infamous association was with Yeats, who had long been in love with her mother. In 1916, in his fifties, Yeats proposed to the 22-year-old Iseult who refused his advances. He had known since she was four and often referred to her as his darling child. Many Dubliners suspected that Yeats was her father.
In 1920, she eloped to London with 17-year-old Irish-Australian Francis Stuart, who would become a respected writer. The couple later married. Their first child, Dolores, died in 1921 of spinal meningitis while three months old. The couple had two other children, Ian and Catherine.
Iseult was not acknowledged as her mother's daughter in Maud Gonne's will when Gonne died in 1953. Iseult died a year later.
- Bolger, Dermot. "A life less ordinary: The tale of Maud Gonne's daughter". The Sunday Business Post, 26 December 2004. Retrieved on 20 June 2007.
- Amanda French, "A Strangely Useless Thing: Iseult Gonne and Yeats," Yeats Eliot Review: A Journal of Criticism and Scholarship 19:2 (2002): 13-24
- Foster, 331-2
- Deirdre Toomey, ‘Stuart , Iseult Lucille Germaine (1894–1954)’, Oxford Dictionary of National Biography, Oxford University Press, 2004
- R. F., Foster. W. B. Yeats: A Life, Vol. I: The Apprentice Mage. New York: Oxford UP, 1997. ISBN 0-19-288085-3.
- Letters to W.B. Yeats and Ezra Pound from Iseult Gonne: A Girl That Knew All Dante Once; Palgrave Macmillan, 2004; ISBN 1-4039-2134-2, ISBN 978-1-4039-2134-5
- French, Amanda. " 'A Strangely Useless Thing': Iseult Gonne and Yeats." Yeats Eliot Review 19.2 (Aug 2002) 13-24.
- Jordan, Anthony J. ' Yeats-Gonne-MacBride Triangle' , Westport Books 2000. ISBN 0-95-244474-7.
- Paseta Senia, 'Irish Nationalist Women 1900-1918' Cambridge University Press 2013.