Here's a breakdown:
* Hero's apparent "death": The play's climax sees Hero publicly accused of infidelity and "dying" from the shock. This event is deeply tragic and seemingly seals the doom of the lovers.
* Don John's triumph: The villain, Don John, who orchestrated the whole scheme, celebrates his success, believing he's ruined Claudio and Hero's lives. This seems like a genuine victory for him.
* The twist: The audience knows that Hero's death is fake and the accusations false. This creates a sense of tension and anticipation for the truth to be revealed.
* The sudden change: The act ends with the arrival of the messenger, bringing news of Claudio's remorse and the revelation of Hero's innocence. This joyous turn of events, occurring after the initial tragedy, is ironic.
* Don John's downfall: The news of Hero's innocence also exposes Don John's deceit and leads to his arrest. This sudden shift from triumph to utter defeat is deeply ironic, highlighting the swiftness of fortune's wheel.
In essence, the irony lies in the fact that what appears to be a tragic end for Hero and Claudio turns out to be a catalyst for their happiness, while Don John's perceived victory leads to his immediate downfall.
This twist, with its dramatic reversals, underscores the theme of misunderstanding that drives the play's plot. The act ends on a note of hope and optimism, but with a sense of unresolved tension, setting the stage for the final resolution in Act IV.