In Act 1, Scene 5 or Macbeth, where is the irony in "Thou'rt mad to say it"?

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malibrarian eNotes educator| Certified Educator

The irony is that Lady Macbeth, upon hearing that the king, Duncan, is coming to her home that evening, would accuse the messenger of being mad to tell her this news.  She has already stepped onto the track of madness by her plans to have Macbeth murder Duncan so they can ascend the throne together; yet she is telling this poor messenger that he is mad to tell her Duncan is on his way to her house. There is more irony because Lady Macbeth continues to descend further into madness as a result of Duncan's ill fated choice to honor Macbeth by staying at Dunsinane castle.