Hynes was a member of the Ancient Guild of Incorporated Brick and Stone Layers.
The union's secretary, Dick Carroll, was shot in 1916 for his part in the Easter Rising, and Hynes succeeded him.
A less public figure than many of his contemporaries, he devoted much of his time to committee work, in negotiations, with the Irish Trades Union Congress, and at the International Labour Organization.
[1] Hynes was active in the Labour Party, and was elected to the Dublin Poor Law Union in 1920, a year in which Sinn Féin won the vast majority of seats.
[1] In his spare time, Hynes studied Gaelic culture and the Irish language.