In Part 1, the firemen's destruction of the woman's house is described with metaphorical language and characterized by situational irony.
Next thing they were up in musty blackness swinging silver hatchets at doors that were, after all, unlocked, tumbling through like boys all rollick and shout. “Hey!” A fountain of books sprang down upon Montag as he climbed shuddering up the sheer stairwell. How inconvenient! Always before it had been like snuffing a candle.
This passage is a great example of Bradbury's writing style, which is full of literary devices and especially metaphorical language. For instance, a simile compares the firemen to boys playing, tumbling, and shouting, which indicates to the reader that the firemen find this fun. Just as was true for Guy at the beginning of the book, the firemen take pleasure in the destruction and burning. A metaphor describes the mass of books as a fountain, as if the flood of knowledge is overwhelming or attacking Guy. This difficulty causes Guy to reflect that "always before it had been like snuffing out a candle," another simile that further complicates the fire symbolism in the novel. It would be easy to assert that fire is, or symbolizes, a bad thing in Fahrenheit 451, but this is not true for all literal or metaphorical mentions of flames. Pay special attention to the metaphorical mention of candles, as well as fire's cleansing or rebirthing potential.
The situational irony comes from the unnecessary violence the firemen use out of habit, or because they enjoy it. They hack down doors "that were, after all, unlocked."
In Part 1, the firefighter's handbook contains an ironically incorrect allusion to early American history.
Stoneman and Black drew forth their rule books, which also contained brief histories of the Firemen of America, and laid them out where Montag, though long familiar with them, might read: Established, 1790, to burn English-influenced books in the Colonies. First Fireman: Benjamin Franklin.
Readers likely know this is all false, especially American readers, who should be familiar with the unique free speech protections in the First Amendment that directly contradict the censorship in Fahrenheit 451. There is special irony in the reference to Benjamin Franklin, who was in fact a newspaper printer, a writer, a proponent of free speech, and someone who valued knowledge and debate. It is true, however, that Franklin founded Philadelphia's first fire department, along with a library and a college that is now known as the University of Pennsylvania. It's hard to think of a historical figure who would have been more opposed to the sort of censorship the firemen are undertaking!
The ironically false framing of these allusions also subtly indicates what happens in a censored society. When only certain viewpoints or claims can be circulated, discovering the truth, or even simply differing perspectives, can be difficult or impossible, and it will begin to seem as if the world has always been, and must always be, the way it is right now.
In Part 3, when Guy realizes the fire truck has stopped in front of his house, his wife runs out into a taxi in an ironic moment.
She shoved the valise in the waiting beetle, climbed in, and sat mumbling, “Poor family, poor family, oh everything gone, everything, everything gone now. . . .”
Mildred turned Guy in, and she now leaves the house where they once lived together. The situational irony lies in the fact that Mildred says "poor family" after putting her husband's life in danger and effectively ending their marriage, but she is not talking about Guy! Instead, she is talking about her television screens, which will be destroyed in the house fire. Recall that Mildred described the characters in the television programs as her "relatives" and "family" because she was so attached to them; she would even participate in their conversations. Mildred betrayed her actual family by calling in a tip on Guy, but she has displaced her affection and love onto the television show characters and newscasters. They are all she cares about as she runs away from her former home.
This particular ironic moment is situational irony because the reader would expect Mildred to have some guilt about turning in her husband, even if she believes she has done the right thing. But instead, her concern is for a "family" that the reader knows is the artifice of compelling and addictive mass media.