She joined Maud Gonne’s organisation Inghinidhe na hÉireann ( Daughter’s of Ireland) under the English version of her name Jennie Flanagan and in her spare time taught Irish in the Gaelic League offices on Parnell Square, Dublin. One of her students was Éamon de Valera, then a teacher of mathematics.
They married on 8 January 1910 and had five sons, Vivion, Éamon, Brian, Ruairi, Terence and two daughters Máirín and Emer. Tragically their son Brian, was killed in a horse-riding accident in the Phoenix Park, Dublin in July 1935. A grief-stricken Sinéad was never able to bring herself to visit her son’s grave in Glasnevin.
After his role in the failed Easter Rising in Dublin in 1916, Sinéad saw very little of her husband. While De Valera was in America raising funds and awareness of the Irish fight for independence, Sinéad lived quietly in Greystones, Co Wicklow with her family. Finding it hard to make ends meet, Michael Collins regularly travelled to Greystones on his bicycle, without detection from the British forces, to bring her money and supplies.
Having not seen her husband for some time, in 1920 Sinéad was smuggled into America on a false passport.
In 1932, Éamon de Valera became head of the government in Ireland and Sinéad began writing stories for children in both English and Irish. She could recite poetry at will, translated fairy stories for children from Irish into English and wrote plays as well as poems for children. Among her works were plays such as Cluichidhe na Gaedhilge (1935) and story collections such as The Emerald Ring and Other Irish Fairy Stories (1951), The Stolen Child and Other Stories (1961), The Four-leafed Shamrock (1964) and The Miser’s Gold (1970).
During her husband’s 14 years as President of Ireland, Sinéad appeared in public on rare occasions, but regularly attended children’s drama competitions in Irish and at Gaelic League functions.
Sinéad de Valera died on 7 January 1975, at the age of 96, the day before what would have been the De Valeras’ sixty-fifth wedding anniversary. Éamon de Valera died just under eight months later, on 29 August 1975, aged 92. The couple are buried together, along with their son Brian, at Dublin’s Glasnevin Cemetery.