Where is situational irony in the "The Necklace" and how does this convey the author's message?

In situational irony, the outcome of an event is the opposite of what is intended.
Monsieur Loisel intends to bring his wife happiness through the invitation to the ball, which was hard for him to come by, as well as giving her 400 francs for a new dress and advising her to borrow a piece of jewelry from her friend Madame Forestier. Madame Loisel hopes to make an impression which will perhaps lead to more invitations and more of the high life she craves.
Madame Loisel is a triumph at the ball. However, rather than leading her up the class ladder, the ball, ironically, leads her to a lower standard of living. She loses the diamond necklace she borrowed, and, from having to borrow money to replace it, the couple becomes poor repaying the loan. An event that was supposed to bring happy memories transforms into a source of misery.
Even more ironically, the Loisels didn't have to suffer at all, because the diamond Mathilde borrowed was a cheap fake. This final irony conveys the author's message that Mathilde attributed too much value to externals, like jewels, and was unable to discern where true worth lay.