What does cladius hear from polonius, rosencrantz and guildenstern that delights him? What is the dramtic irony in this?

Rosencrantz and Guildenstern have not yet discovered the source of Hamlet's distemper. They do however point out that news of the players arriving made Hamlet very happy. The King likes this and resolves to watch the play with Hamlet. The audience knows the king watching the play is part of Hamlet's plan for revenge.