Born in Tuamgraney, County Clare, she was educated locally at Scarriff, and in Loughrea, County Galway. She went on to become a Licentiate of the Pharmaceutical College of Ireland. From the publication of her first book, The Country Girls, O'Brien achieved a notoriety in Ireland by the relatively free treatment of sexual themes, which brought her into conflict with the Catholic Church, and got many of her books placed on the list of books banned by the Irish Censorship Board. Several of her books were successfully filmed. These circumstances have ensured that her name is still familiar in Ireland, even to those who may not read her books.
O'Brien's first three novels follow the lives of 'Kate' Brady and 'Baba' Brennan, from their narrow home communities in rural Ireland, through their arrival and life in Dublin, and on to London. This trajectory was a familiar one in the Ireland of the sixties, marked by bigotry, prudishness, poverty and emigration. Much of her later writing is similarly concerned with the status of women in society, and how individuals can achieve happiness and fulfillment given the social constraints which bind them.