Let us remind ourselves of the definition of situational irony. Situational irony is when there is a sudden, unexpected reversal of what we expect to happen in a story. The classic example of this is "The Gift of the Magi," when there is a sudden, shocking ending as Jim and Della that they have both traded the possessions that were dearest to them to purchase gifts that now cannot be used by the other.
If we think about this concept in terms of this novel, I would argue that the ending of the story is an example of situational irony. Having finally escaped the mechanical hound and found a group of Book People who he can join, Montag is looking forward to a life of hidden opposition and remembering texts. Instead, both he and the reader are shocked by the sudden destruction of the city from which he has just fled:
The bombardment was to all intents and purposes finished once the jets had sighted their target, alerted their bombardier at five thousand miles an hour; as quick as the whisper of a scythe the war was finished. Once the bomb release was yanked, it was over.
The shock with which this rapid and sudden destruction of the city occurs is as much of a surprise for Montag as it is for us. Now the group will not have to operate in secret, and can be part of the phoenix rising from the ashes that Granger remembers. It is a sudden twist in the plot that takes us by surprise.