It's not clear how Hayes, who joined the National Library with a degree in languages from Trinity College Dublin (TCD) was identified by army intelligence as a codebreaker.

[3] Hayes made significant progress in breaking the code and in February 1941, at the behest of Éamon de Valera, he was given an office and three lieutenants to decode wireless messages being covertly transmitted from a house in north Dublin owned by the German legation.

[citation needed] As far as Schütz and the other German spies whose cells he entered during the Emergency knew, their quiet-spoken and polite interrogator was "Captain Grey", a mysterious military figure always accompanied by another intelligence officer.

[citation needed] Even Günther Schütz went to his grave in 1991 still believing that it was another man, Commandant de Butléir, who was responsible for identifying his microdot codes – the first cryptologist in the world to do so.

Günther Schütz was dropped into Ireland on 12 March 1941 to make contact with IRA members sympathetic to the German cause and to transmit weather reports[3]: 159  back to his handlers in Hamburg.

In all, 30 pages of operating instructions as well as extensive lists of names and addresses of Nazi sympathisers in the Republic were secreted in random characters in newspaper cuttings that the German intelligence agent was carrying when he was picked up by gardaí in Wexford in 1941.

"[citation needed] Hayes had some success decoding cable messages, but it was working on complex letter based ciphers that he demonstrated his brilliance as a code breaker.

A similar cipher had already baffled cryptologists at Bletchley Park, the headquarters of British code-breaking activity, but Hayes finally identified a system of decoding it[3]: 237  based on a sequence of rotating keywords.