Marcus Beresford Armstrong was born on 19 May 1859 in Ballydavid House, Woodstown, County Waterford. He was the eldest child and only son of Captain William Armstrong (1826-1889) and Catherine Clark. Like his father, he chose a military career and on 13 March 1886 was granted the rank of Captain in the 8th Brigade, North Irish Division, of the Royal Artillery. He married on 11 April 1888 Rosalie Cornelia Maude of Lenaghan Park, Enniskillen, County Fermanagh, and lived with her in Sligo, where their four children were born. In 1899, Marcus inherited the estates of Moyaliffe Castle, County Tipperary, and Chaffpool House, County Sligo, from his first cousin once removed, Captain Edward Marcus Armstrong, who had died without immediate family. He sold the Chaffpool estate in 1904-1905 and moved his family to Moyaliffe Castle. However, some years after the move Marcus and Rosalie became estranged. In 1914, when the war broke out, Marcus was living at Moyaliffe Castle on his own. In addition to running the family farm, he was an enthusiastic sportsman, with an abiding interest in shooting, fishing and horse racing. He was known to his friends as Mark, and to his children as Boss, Sir, or Poppy.