Born either Jane or Janice Flanagan in Balbriggan, County Dublin (she altered the spelling of her name to the Irish equivalent—Sinéad—later in life), she trained as a teacher and took up her first post in a national school in Dorset Street, Dublin. In her spare time she taught Irish with the Gaelic League in Parnell Square. One of her Irish students was Éamon de Valera, then a teacher of mathematics.
On 8 January, 1910, they were married, although she was four years older than her husband. Together they had five sons, Vivion, Éamon, Brian, Ruairi and Terence (Terry), and two daughters, Máirín and Emer. During her husband's 50 years in public life she played little or no public role. Following the Easter Rising in 1916 she saw little of her husband. In 1932 de Valera became head of the government, and Sinéad de Valera started writing stories for children in both English and Irish. Though she kept to the background as far as public matters were concerned, Sinéad was a highly political person. Rumours differ, however, as to whether she was in fact more moderate or more radical than her husband. During her husband's fourteen years as President of Ireland she appeared in public on only very rare occasions.
She wrote a book, Scealta Sí, which retold in her own words twelve traditional Irish stories.
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. Her husband Éamon died nearly eight months later, on 29 August, 1975.