The famous science fiction novel Fahrenheit 451 by Ray Bradbury tells of a future society in which firemen are employed to burn books instead of put out fires. The protagonist of the story, Montag, is a fireman, but at the same time he hides books and reads them, and he eventually comes to oppose the book-burning that society endorses.
Irony in literature occurs when things in a story are actually very different than what they appear to be. Irony can be verbal, when someone says something different than what they actually mean, or situational, when things happen that are different from what readers expect to happen. At the beginning of Fahrenheit 451, there are two significant examples of situational irony.
Firstly, Bradbury introduces the fireman Montag not as someone dedicated to putting out fires, as readers would expect, but instead as someone who loves to burn things. The first line in the book says, "It was a pleasure to burn." This is not a thought that we would think a fireman would have. As the story goes on, though, we understand that this is a society that considers learning perverse and books dangerous, and that's why firemen are employed to start fires instead of put them out.
The other major irony has to do with Montag's love of burning things. We understand right away on the first page that he loves his job, but at the same time, as soon as he gets home, he looks up at a ventilator grille in his hallway and considers that "something lay hidden behind the grille, something that seemed to peer down at him now." Later, we learn that Bradbury is referring to a trove of books that Montag has hidden up there. The irony is that as a fireman he seems to love his job of burning books, but at the same time he recognizes the value of books and seeks to save at least some of them.