The best example of irony is found in the play Julius Caesar when Mark Anthony says,  "And Brutus is an honorable man."  We all know that he actually means the opposite of what he is saying. 

There are three kinds of irony:

verbal irony - when the writer says one thing but means another.  (The example above from Julius Caesar is verbal irony.)

situational irony - when the character expects one thing to happens but something else actually happens instead

dramatic irony - when the reader knows something that the character doesn't