Duncan entering the castle
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I suppose the irony is dramatic. Lady Macbeth sets out a plan for she and her husband to present themselves as loving subjects and yet they are underneath, as the audience knows, ready to murder the king. This is dramatic irony because we know something that Duncan does not. What's more, an extra level of irony is that Macbeth actually does love Duncan, yet he's torn by the ambition being spurred on by his "sexless" wife.
As Duncan enters Macbeth's castle he says that he feels the castle and the air to be gentle and pleasant,
This castle hath a pleasant seat; the air
Nimbly and sweetly recommends itself
Unto our gentle senses.
The irony here is that Macbeth and his wife plan to stab him about three hundred times!