What kind of irony is exemplified by the outcome in "The Gift of the Magi?"
Della sells her hair for twenty dollars in order to buy Jim a platinum chain for his watch. She goes home, curls what is left of her hair, and waits for Jim to come home. At this point, we (readers) have no idea what Jim has for Della. So, it is a shock to learn that he has purchased "The Combs" for her. This is an example of situational irony because this is when the outcome contradicts what is expected. It is ironic for the reader and for the characters because they could not have expected this result.
A perceptive reader might then guess that Jim sold his watch to buy the combs. In this case, it would be an example of dramatic irony because this occurs when the reader knows something a character does not. In this case, if the reader correctly assumes that Jim sold his watch, the reader knows something that Della does not yet know.
However, if the reader does not make this assumption, it is still situational irony because, again, the result is contrary to what was expected. In this case, the reader does not foresee this second result. Likewise, neither Della nor Jim expected their gifts to be rendered useless as a result of their mutual generosity. The irony is created out of the situation in which the result is unexpected.