What is ironic about the speeches of Duncan and Banquo as they approach Macbeth's castle?

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As the approach Macbeth's castle, Duncan and Banquo both remark about what a pleasant place it is. They gush about its beauty, with Banquo remarking that even the birds, who are nesting in its crannies, recognize how "delicate" the air is. In particular, they remark that the air seems very healthy there (a common concern in a time when people still believed that foul air caused disease.) Of course, it does not turn out to be such a healthy place for Duncan, who is murdered there, or for Banquo, whose ghost will appear to Macbeth there after he is murdered while riding with Fleance. The audience, of course, already recognizes this irony, as they have become aware of Macbeth's ambitions and of Lady's Macbeth's determination to bring them to fruition. The irony, and the Macbeth's duplicity, is further underscored in the scene when Lady Macbeth and Duncan warmly greet each other at the entrance to the castle.