Deevy is born on January 21, 1894 in Waterford. She is the youngest of 13 siblings, all girls. Her mother is Mary Feehan Deevy and her father is Edward Deevy who passes away when she is two years old.
Deevy attends the Ursuline Convent in Waterford and in 1913, at the age of 19, she enrolls in University College Dublin, to become a teacher. However, that same year, she becomes deaf through Ménière’s disease and has to relocate to University College Cork so she can receive treatment in the Cork Ear, Eye, and Throat Hospital, while also being closer to the family home. In 1914 she goes to London to learn lip reading and returns to Ireland in 1919. She starts writing plays and contributing articles and stories to the press around 1919.
Deevy’s return to Ireland takes place during the Irish War of Independence and this heavily influences her writing and ideology as she is heavily involved in the nationalistic cause. She heavily admires Constance Markievicz and joins Cumann na mBan, an Irish women’s Republican group and auxiliary to the Irish Volunteers.
In 1930 Deevy has her first production at the Abbey Theatre, Reapers. Many more follow in rapid succession, such as In Search of Valour, Temporal Powers, The King of Spain’s Daughter and Katie Roche, the play she is perhaps best known for. Her works are generally very well received with some of them winning competitions, becoming headline performances, or being revived numerous times. After a number of plays staged in the Abbey, her relationship with the theater sours over the rejection of her play, Wife to James Whelan in 1937.
After Deevy stops writing plays for the Abbey, she mainly concentrates on radio, a remarkable feat considering she had already become deaf before radio had become a popular medium in Ireland in the mid-to-late 1920s. She has a prolific output for twenty years on Raidio Éireann and on the BBC.
Deevy is elected to the prestigious Irish Academy of Letters in 1954, as a recognition to her contribution to the Irish theater. Her sister, Nell, with whom she had lived in Dublin, dies in the same year, so she returns to Waterford. She becomes a familiar figure in Waterford as she cycles around the city on her “High Nelly” bike.
When Deevy’s health begins to fail she is eventually admitted to the Maypark Nursing Home in Waterford. She dies there on January 19, 1963, at the age of 68, two days before her birthday.