Keane is born Mary Nesta Skrine on July 20, 1904 in Ryston Cottage, Newbridge, County Kildare. Her mother is a poet who writes under the pseudonym Moira O’Neill. Her father is a fanatic for horses and hunting. She grows up at Ballyrankin House on the banks of the River Slaney, a few miles southeast of Bunclody, County Wexford and refuses to go to boarding school in England as her siblings had done. She is educated by her mother, governesses, and at a boarding school in Bray, County Wicklow. Relationships between her and her parents are cold and she states that she had no fun in her life as a child. Her own passion for hunting and horses is born out of her need for fun and enjoyment. Reading does not feature much in her family, and, although her mother writes poetry, it is of a sentimental nature, “suitable to a woman of her class.”
Keane claims she had never set out to be a writer, but at seventeen she is bed bound due to suspected tuberculosis, and turns to writing out of sheer boredom. It is then she writes her first book, The Knight of Cheerful Countenance, which is published by Mills & Boon. She writes under the pseudonym “M. J. Farrell,” a name over a pub that she had seen on her return from hunting. She explains writing anonymously because “for a woman to read a book, let alone write one was viewed with alarm: I would have been banned from every respectable house in County Carlow.”
In her teenage years Keane spends much of her time in the Perry household in Woodruff, County Tipperary. Here she befriends the two children of the house, Sylvia and John Perry. She later collaborates with John in writing a number of plays. Among them is Spring Meeting, directed by John Gielgud in 1938, and one of the hits of the West End that year. She and Gielgud become life long friends.
It is through the Perry family that Keane meets Bobby Keane, whom she marries in 1938. He belongs to a Waterford squirearchical family, the Keane baronets. The couple goes on to have two daughters, Sally and Virginia.
Keane loves Jane Austen, and like Austen’s, her ability lay in her talent for creating characters. This, with her wit and astute sense of what lay beneath the surface of people’s actions, enables her to depict the world of the big houses of Ireland in the 1920s and 1930s. She “captured her class in all its vicious snobbery and genteel racism.” She uses her married name for her later novels, several of which (including Good Behaviour and Time After Time) have been adapted for television. Between 1928 and 1956, she writes eleven novels, and some of her earlier plays, under the pseudonym “M. J. Farrell.” She is a member of Aosdána.
Keane’s husband dies suddenly in 1946, and, following the failure of a play, she publishes nothing for twenty years. In 1981 Good Behaviour comes out under her own name. The manuscript, which had languished in a drawer for many years, is loaned to a visitor, the actress Peggy Ashcroft, who encourages Keane to publish it. The novel is warmly received and is short-listed for the Booker Prize.
Following the death of her husband, Keane moves to Ardmore, County Waterford, a place she knows well, and lives there with her two daughters. She dies on April 22, 1996 in her Cliffside home in Ardmore at the age of 91. She is buried beside the Church of Ireland church, near the centre of the village.