In Guy de Maupassant's short story "The Necklace," the dramatic irony—if, in fact, it is dramatic irony—rests in Mathilde's desire to have more than she does, and the lengths to which she is willing to go to achieve what she wants.
...(the most important type for literature) involves a situation in a narrative in which the reader knows something about present or future circumstances that the character does not know.
Mathilde Loisel is of the lower-middle class. She has the appeal of a young woman who would have done very well for herself had she been born to the upper-class.
The girl was one of those pretty and charming young creatures who sometimes are born, as if by a slip of fate, into a family of clerks.
…she was unhappy as if she had really fallen from a higher station.
It does not occur to Mathilde that she should be satisfied with her life. Her husband works hard to provide for her and make her happy. He does not complain. For example, when he sits down to dinner, he is delighted with what has been prepared. She can only notice that the tablecloth on which their dinner rests has been used three days in a row.
Mathilde does not have all that she believes she should have. Her husband is very patient and kind. However, no matter what he does, it is not enough for Mathilde. She is forever unhappy—critical and unrealistic—and always grasping for more.
One evening her husband comes home from work with an invitation to a ball being given by the company for which he works. He tells her it was difficult to get the invitation; he expects her to be beside herself with joy. Rather than responding with pleasure, she starts to cry, noting that she has nothing to wear. Her husband offers up money he had saved, so she can buy a dress, even though he had hoped to buy something for himself.
As the day approaches, Mathilde is still not pleased. Now she complains that she has no jewelry. Her husband suggests that she could adorn herself with flowers that are in bloom. Mathilde is not interested in flowers: she must have jewelry so she does not (as she sees it) look like a pauper among the rich.
Mathilde's husband suggests she borrow jewelry from her friend:
Go look up your friend, Madame Forestier, and ask her to lend you some jewels.
Mathilde is delighted with the suggestion. Upon visiting, she asks if she might borrow something for the ball. Her friend is glad to help. She offers up her jewelry box, but Mathilde is not satisfied.
She kept asking:
"Haven't you any more?"
Finally Mathilde sees a black velvet box containing a diamond necklace; she asks to borrow that piece. Madame Forestier agrees and Mathilde takes it home.
The reader is aware by now that Mathilde will be going to the ball pretending to be something she is not. She has a lovely dress and a beautiful necklace. Only these things bring her joy. She does not appreciate her husband's work and enduring patience. She does not consider herself blessed to have a home and the opportunity to be well-provided for—even having a servant and being able to attend the theater. She is shallow and ungrateful.
They go to the ball and she has a lovely time. When they return home, she is sad that the evening is over. Upon preparing to remove her dress, she notices that the necklace is gone. They search everywhere. Her husband tries to find the carriage they took home, and even visits the police station. They will need almost forty thousand francs to replace it. Monsieur Loisel has some money that his father left him: the rest he will need to borrow.
And so the money is borrowed. They have to release their servant and move to a very small apartment. Monsieur Loisel takes on extra work and Mathilde does all the housework—even the heavy duties—herself. All this happens because she was not satisfied with what she had. Her husband ages more quickly than his actual years. Even Mathilde begins to look "strong, hard and rough." They continue living so for ten years. During this time Mathilde wonders:
What would have happened if she had not lost that necklace? Who knows? Who knows? How strange and changeful is life! How small a thing is needed to make or ruin us!
When the debt is finally paid, Mathilde and her husband take a walk one Sunday and run into her friend Madame Forestier, still young and beautiful. She does not recognize Mathilde at first because Mathilde is now so hard-looking. Mathilde explains what had happened with the necklace and how they had paid back all the money for the jewels.
And she smiled with a joy that was at once proud and ingenuous.
Madame Forestier is amazed, and it is in her comment that we see the depth of the story's irony:
Oh, my poor Mathilde! Why, my necklace was paste! It was worth at most only five hundred francs!
If there is dramatic irony in the story, it is that Madame Loisel pretends to be what she is not. In this way, the reading audience is aware that Mathilde is doing nothing more than playing dress up. It is also ironic (though it is not dramatically ironic) that the necklace is also not what it seems to be. Both Mathilde and the jewels appear to be beautiful and of great worth. Mathilde is poor and the necklace is worthless—made of paste. As with Mathilde on that most special evening, appearances were deceiving. The reader is aware that Mathilde was not who she purported to be. She was a woman of a class lower than the one to which she wishes she had been born.
There is also situational irony:
Situational irony...is a trope in which accidental events occur that seem oddly appropriate, such as the poetic justice of a pickpocket getting his own pocket picked. However, both the victim and the audience are simultaneously aware of the situation in situational irony--which is not the case in dramatic irony.
Because of Mathilde's endless desire to be rich, her actions cause her and her husband to lose all they have. In an example of true "poetic justice," the imagined poverty Mathilde believes she lives in at the start of the story becomes a reality when the necklace is lost. Had Mathilde been satisfied with her circumstances (as she wondered to herself), how differently life might have turned out to be.