This was characterised by meticulous archival research, and it anticipated by sixty years the much vaunted New British History of the late twentieth century by tracing the interconnections between events in England, Ireland, and Scotland.
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.
His research, long-standing personal interest in the military, and his curatorial experience, helped form an expert knowledge of historical Irish warfare.
The committee was also to further offer guidance and oversee progress of the Bureau in co ordination with the Ministry of Defence: it subsequently expressed concerns about the state's role and methods in the collection of statements.
During the 1940s and 1950s, Hayes-McCoy became involved in a number of paratheatrical events of national significance one of which – the "Pageant of St.Patrick" for which he wrote the script (An Tóstal 1954) – was realised on an immense scale.
In that capacity, he collaborated in 1955 and 1956 with Micheál Mac Liammóir and Denis Johnston on their scripts for pageants on St.Patrick and on the Táin Bó Cuailgne, at times finding it difficult to square the historical liberties taken by these artists with his own role.
In 1959 Hayes-McCoy succeeded to the chair of his former history professor at UCG with the full remit of lecturing (in English), administering examinations to undergraduates, and supervising postgraduate theses – among those of his students who continued in the field of history were Nicholas Canny, Martin Coen, Patrick Melvin, Peter Toner, Tony Claffey, and Breandán Ó Bric.
The ultimate failure of the campaign informed Hayes-McCoy's regret, expressed a year later, that Ireland was forgetful about its past and that "we don't bother to find out about it or to maintain our ancient heritage", and, on a perceived spirit of conformity: "take my own city of Galway, it is now more prosperous than it was, but it is no longer distinctive.
I do not believe that it is essential for progress that we should lose our heritage"[citation needed] While at one time member and secretary of the London Sinn Féin office (Roger Casement Cumann, 1935), and informed by a pride of country and place, Hayes-McCoy's professional and private outlook were marked by a distrust of nationalism or of any antagonising national agendas compromising genuine scholarship.
Among his papers in the James Hardiman Library, NUI Galway are c. 40 items with predominantly maritime subjects, and he had a special regard for the history of ships, and a romantic liking of the sea.