How did O'Casey arrive at this famous sentence?

Boyle speaks these words many times throughout the play. They reflect his deterministic world view and the assumption that man can do nothing to fight against the chaos ("chassis") that has ensued. Instead, Boyle escapes through fantasies and drink. His attitude contrasts with Juno's assumption of free will: "Ah, what can God do agen the stupidity o' men!" (70.) The appearance of this line both at the beginning and the end of the play suggests that little has changed, at least for Boyle.