What is the irony at the end of Part 2 of Fahrenheit 451?
At the end of Part Two, Montag is at work in the firehouse when a "special case" comes in. Beatty, Montag and the other firemen get into the vehicle and begin driving across town, just as they would for any other case. When the vehicle stops, however, Montag realizes that they are standing in front of his own house. This is not a mistake: tonight, his house is the firemen's target.
It is ironic that a fireman should set fire to his own house since he is the living embodiment of the law against reading books. Remember that it is his job to burn illegal copies of books, not to read them. Montag has, therefore, undergone significant changes: at the beginning of the novel, burning books was a "pleasure" to him but, by the end of Part Two, he is himself the target of the fireman's hose and he cannot deny his own guilt.
Being a fireman Montag puts himself in a compromising position by possessing all these books. He ends up confiding in his wife, which was a dire mistake because her loyalty is to the letter of the law and she turns him in.
Since it is the job of the fireman to burn books (start fire rather than put it out) at the end of part 2 it is ironic that the fireman have to go to the house of one of their own to burn books. Firemen should be setting the example for others in this society and instead Montag is a lover of literature.
The irony is that the firemen, including Montag, arrive at Montag's house. He has been turned in for possessing books. Of course, it is his own wife, Mildred, who turns him in. Now Beatty makes Montag burn not only his house but all of the books and places him under arrest.