The key moment of dramatic irony in the story is the ball, in which the reader knows that Mathilde has dressed up in imitation of a wealthy woman, when she in fact is not. This fact is known to the reader, though not to the other characters in the scene:
Madame Loisel was a grand success. She was lovelier than any other woman […]. All the men gazed at her, asked for her name, and tried to get introduced. All the cabinet attachés wanted to waltz with her. The minister noticed her.
Though this story is told from a third-person limited point of view, it focuses intimately on Mathilde’s interiority. So, the reader is aware that the interest of the men in the room may be exaggerated and distorted by Mathilde’s ego, unbeknownst to her (a double irony). The result is that a kind of narrative tension runs through this scene: the reader wonders just how effectively Mathilde has truly disguised herself as a wealthy woman. What's more, readers might wonder how Mathilde will return to her normal life now that she has—apparently—experienced what it feels like to have wealth and status.
Ultimately, the reader is also aware that the interest that these men take in Mathilde probably will not change her life. Realistically, it cannot: Mathilde’s identity as a wealthy society woman is borrowed for a night, and it will be returned with the necklace in the morning. However, Mathilde clings to the fantasy of wealth she feels she has attained, and she seems to invest great importance in retaining their attention. The result is that Mathilde’s self-absorption and greed are laid bare for readers to see.
