There is situational irony. The readers and Mathilde do not expect for the necklace to be fake. Also, the first several paragraphs are spent describing Mathilde's desire to be rich. Although she is not as wealthy as Madame Forestier, the Loisel's are not dirt poor; they do have a maid and a cook. But because of Mathilde's desire to be rich or to seem rich, they end up actually becoming poor.