Rosalie Cornelia Armstrong was born on 26 April 1868 at Lenaghan Park, Enniskillen, County Fermanagh. Her father, Maurice Ceely Maude (1820-1904), was a grandson of Cornwallis Maude (1729-1803), first Viscount Hawarden, of Dundrum, County Tipperary. Her mother, Elise Maria Wehren (d. 1895), was from Berne, Switzerland. Rosalie’s parents had ten children, of whom one died as a child.
Rosalie was her parents’ fourth child and eldest daughter. She married Captain Marcus Beresford Armstrong on 11 April 1888 and had a son and three daughters. Sometime after the family’s move to Moyaliffe Castle, Rosalie’s relationship with her husband became strained, and the couple decided to lead separate lives. At the start of the war, Rosalie and her three daughters were living in a boarding house at 14 Trinity Crescent, Folkestone. She was known to her friends as Rosalie, Rosie, Mrs Armie or Mrs Pat. Her children called her Mus, Muz, or Motherbird.