A lot of the humour and tragedy in the play is created through O’Casey’s use of dramatic irony.
When Joxer sympathises with Boyle about his stolen bottle of stout - hyperbolising that "man’s inhumanity to man makes countless thousands mourn" - it is all the funnier because the audience are aware that Joxer has stolen the stout himself.
It is also a source of humour when Joxer and Boyle treat each other as best friends, despite the audience knowing that behind backs they frequently criticise each other.
There is comedy too in Boyle’s cowardly fear of his wife. He talks loudly to Joxer of his superiority in the marriage when Juno is not on stage, only to back down meekly as soon as she confronts him.
However, O’Casey uses dramatic irony in a different way in the final scene where Joxer and Boyle enter the stage, "both of them very drunk".
Despite their banter and song lyrics - which seemed funny and playful in Act I - there is an atmosphere of sadness and horror as the audience knows of Johnny’s death while Boyle does not.
His drunken garbling about "Volunteer Butties" and people who "died for … Irelan’" seem hollow and poignant now.
His final assertion that "‘th’ whole worl’s … in a terr … ible state o’… chassis!" seems a much gloomier truth than when he said it in Act I.