Gerard Anthony Hayes-McCoy was born 15 August 1911 in Galway, of Thomas Hayes McCoy and Mary Kathleen Hayes McCoy (née Wallace). His grandfather Thomas Hayes McCoy had been a Dubliner who as a child came to Galway in 1834; he was later a well-known Parnellite.

Hayes-McCoy received his early education from the Patrician Brothers, Galway. His earliest notebook of 1927 and a manuscript history of Poland of the same year, now at the National Library, testify to an early interest in history and heritage. From 1928 to 1932 he was a student scholarship holder at UCG, graduating in 1932 with a Bachelor of Commerce, and a Bachelor of Arts, with first-class honours in both, and a specialisation in "History, Ethics, Politics" for the latter

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.

In 1946, he was appointed to a committee of eight historians to advise on setting up The Bureau of Military History - a body established by the Minister of Defence for the creation and compilation of material on the history of the Irish movements for independence, 1913-1921.

On Irish radio and television Hayes-McCoy was most active in the mid-1960s; editing and contributing to Thomas Davis lectures series, writing scripts for a series of thirty children's programmes on all aspects of Irish history, and preparing the television series "Irish battles" and "The long winter".

Among the books he wrote is "A history of Irish flags from earliest times (Dublin 1979)". His collection can be viewed at University College Galway