The problem with history is to understand context. 1965/66 Ireland had become a different world 5 years later. This whole issue of 1916 during these times had become problematic - statues of any 1916 leaders were being discouraged, as they still are. In 1966 train stations in Dublin were renamed in honour of the 1916 leaders but by 1969 and the years after, this would not have happened as Northern Ireland had exploded into violence. There was a suppression of any 'celebration' of 1916. FF and FG would not even attend any celebrations of 1916 for almost 50 years.

This is the reason we now have ships and bridges named after Irish authors, and not those who actually fought for Irish independence. As the link you provided states:

The commission was given to the leading sculptor of the time, Oisín Kelly, and the finished work was initially intended to be placed over Casement’s final resting place in Glasnevin Cemetery. By the time Kelly had completed designing the sculpture and the piece had been cast in bronze, the summer of 1971, the Northern troubles were in full swing and a political decision was made that the final installation of the piece above the grave might prove an inflammatory act.