Irony is a complex concept. Irony is used when there is a significant difference between what the writer is saying, and what the reader believes to be true. Often it is synonymous with sarcasm. Often it refers to a wild fluke of chance, a series of impossible or near-impossible events. Often it is simply a series of events which is wildly different than what we, as the readers, would have expected. In many cases, it's the exact opposite of what is generally believed, or the coexistence of mutually exclusive polar opposites. Irony can take tragic or comic form, but today the latter is presently much more popular than the former.