Despite the darkness of the play, there is a lot of humour in the dialogue and situations that take place. These mainly involve the characters of Jack Boyle and his sidekick Joxer.
O’Casey uses humour to provide moments of relief in the play, but also to emphasise the darkness of the themes and the unpleasant aspects of characters’ personalities.
Joxer and the Captain’s mock-intellectual discussions about books and the nature of the stars and the moon are ridiculous when we see how lazy and feckless they are - especially when juxtaposed with Juno’s hard work to support the family.
We see this too with their use of high-brow references. For example when Joxer philosophises that "man’s inhumanity to man makes countless thousands mourn" after Boyle notices his bottle of stout has been stolen.
This reference to Robert Burns seems like hyperbole for such a trivial incident, and it is all the funnier because we know that Joxer has stolen the stout himself.