 Cosgrave was born at 4 Georges Quay, Dublin, to Patrick Cosgrave, a sail maker, and Mary Whittaker.In 1901 she was living with her younger brother and older sister, Thomas and (Mary) Anne, in Georges Quay and keeping house for them.In 1911 Cosgrave, now a milliner, was living at Tara Street and filled out her census form in Irish.