In the absence of an academic post, Hayes-McCoy became an assistant keeper in the Art and Industrial Division at the National Museum of Ireland (1939–1959), with a responsibility for the Military History, and the War of Independence collections. One of his first tasks was to prepare a standing exhibition on Irish history before 1916. His research, long-standing personal interest in the military, and his curatorial experience, helped form an expert knowledge of historical Irish warfare. This led to his role in co-founding The Military History Society of Ireland in 1949 whose journal "The Irish Sword" he edited (1949–1959). Hayes-McCoy described the vagaries of setting up such a body, its reception, and the historiographical considerations attendant on it, in a paper published posthumously in "The Irish Sword".