What is the irony in "My Last Duchess"?

The irony in "My Last Duchess" is that the last duchess was unaware of her failings in the eyes of the duke, and the duke seems unaware of his cruelty towards the duchess and the failings of his own character. Further, the duchess's quality that so displeased the duke is actually a really lovely and admirable one.

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There are a couple of different types of irony at work in this poem. First, there is dramatic irony, which is created when the audience knows something that a character does not know. We can think of the duke, who narrates the poem, as well as his "last Duchess" as characters. The duchess, it seems, did not realize that she displeased the duke by being so "easily impressed" by the gifts presented to her and the compliments given to her by all and sundry. He wanted her to appreciate him the most, to rank his favor and the honor of being chosen by him above all else, though he refused to tell her this, and so she was unaware of her failing in his eyes. However, we now know and understand this. She was also probably unaware that he "gave commands"—likely to have her killed—so that he could start over with a new wife.

The duke, for his part, seems unaware that his move to have her killed so that he could move on with a new wife is cruel and horrible, as he actually tells the person who is there to broker the contract for his next potential marriage all about it. His attitude toward his last duchess also constitutes an example of situational irony, when there is a discrepancy in expectation and reality in regard to the situation itself. The duchess seemed like such a sweet lady, a person who appreciated every small kindness or compliment, and yet this quality in her is the thing that most bothers the duke. This situation defies expectation.

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