Martin was born at Ross House in Connemara, County Galway, the youngest of sixteen children of James Martin of Ross (1804–1872). The Martin family, a branch of the Martyn family - one of the Tribes of Galway - had settled at Ross by the early seventeenth century, having previously inhabited the town of Galway for some three hundred years. Her father, James, was a Protestant, his grandfather having converted from the Catholic faith in order to retain the family estates under the Penal Laws. Nevertheless, each child of the family was secretly 'baptised' by the family servants, a practice James Martin winked at.