Mhac an tSaoi was born as Máire MacEntee in Dublin in 1922. Her father, Seán MacEntee, was born in Belfast and was a veteran of the Easter Rising, the Irish War of Independence, and of the Anti-Treaty IRA during the Irish Civil War. MacEntee was also a founding member of Fianna Fáil, a long-serving TD and Tánaiste in the Dáil and, . Her mother, County Tipperary-born Margaret Browne (or de Brún), a teacher at Alexandra College, was also an Irish republican. Her uncle Monsignor Pádraig de Brún was one of the most respected scholars of the Irish language in the twentieth century. Her other uncle was the Traditionalist Catholic Cardinal Michael Browne, who was Master of the Dominican Order and, in his later life, a friend and ally of Archbishop Marcel Lefebvre.