The ironic decompression
One might expect Robert to be extremely timid and careful when he isn't fighting vampires, but his life is more complicated than that. He decompresses when he isn't pressed for survival, and he relishes his life in a licentious way, often very drunk, listening to music loudly. This is plainly ironic, because it seems counterintuitive; if he wants to survive, why bring so much attention to his location? Yet, it is part of what makes survive worth it for him in his dire straights.
The dog
The dog who keep Robert company is a tool of dramatic irony, among other things. As a tool of irony, he works to expose Robert to something the reader knows already. When he attaches himself to the dog and then loses the dog to vampires, that is a sign for his loneliness and desperation. He was already lonely and desperate, but now he can't avoid that fact—the emotion is too obvious and painful to ignore any longer. The dog is like Wilson in Cast Away.
The mercy for his wife
Robert couldn't bring himself to kill Virginia, his wife. Instead, he buried her, undead. When she inevitably rose from her slumber to kill him, as any vampire would, he is forced to kill her. The scene is filled with bitter irony and anguish; he is a man killing his own wife because she wants to kill him first. She is his wife, but in a way, she isn't a real person anymore. Her ironic place in the novel helps the reader see the absolute dehumanization of the vampire plague.
Ruth and irony
The character of Ruth is deeply ironic. On the one hand, Robert notices that she seems human, but her human aspects aren't comforting to him anymore. Ironically, they are deeply disturbing, because he has adjusted to a life of pure loneliness. He goes a little bit crazy trying to prove to himself that she isn't a vampire, and she lets him, until he wants her blood. Ironically, he wants access to her blood which makes him seem a little vampiric.
The ending
The ironic twist in the end of the novel is that a greater issue is coming that Robert will not survive. The vampire plague is only the first wave of an invasion that he will not survive, and he shows this to the reader by not hiding; he even takes a cyanide pill to kill himself so the vampires can't have the pleasure of killing him. The final wave of vampires is an absolute machine of death, like the nuclear bombs that start the novel's plot. They aren't concerned with sparing anyone, monster or human.
