Situational irony refers specifically to events where the setup causes an outcome opposite to expectations; it hinges on reversal of expectation rather. In the book, a good example of situational irony is the use of the term "firemen." In present-day society, firemen are public service workers who respond to sudden fires, either environmental or residential, and work with water and chemicals to put the fires out and save lives. However, in the book, it is made clear from the beginning that the firemen are "men of fire," charged with burning books and allowed to kill people who break the law without consequence.
"Established, 1790, to burn English-influenced books in the Colonies. First Fireman: Benjamin Franklin."
RULE 1. Answer the alarm swiftly.
2. Start the fire swiftly.
3. Burn everything.
4. Report back to firehouse immediately.
5. Stand alert for other alarms.
(Bradbury, Fahrenheit 451, Google Books)
This is, of course, ridiculous; there is no reason why fires would be considered a good course of action, especially in regards to books. Also, Ben Franklin was the first firefighter, not an arsonist. However, since the future society is based on collective ignorance, it is imperative that the people be both afraid to read, and distrustful of anyone who thinks differently; the violence of the fire and the public consequences of burning are enough to continue this mindset. In this way, the term "firemen" is reversed and made ironic because it refers to the direct opposite of its initial meaning.