The house burning
The couple spends the entire novel praising the virtues of their suburban life. They're living the dream, so to speak, and they do the domestic things that are expected of them. But they're only deceiving themselves; at the end of the novel, the spontaneously decide the best thing for their marriage would be to burn the house down and leave their horrible cookie-cutter lives behind.
Elaine's affair
Elaine's affair is technically ironic, and not because she was having an affair (Paul could have easily deduced that from their sex life). Elaine's affair is ironic because she is sleeping with her best friend Liz, when Paul thought they were just doing friend things. He didn't even know Elaine was bisexual which is an added layer of dramatic irony.
Paul's affair
Paul is not any better than his wife. He meets an attractive girl at their house party, and before the party ends, they've arranged to have a sexy phone call. This is not the ironic part though—the ironic part is that the girl is Henry's girlfriend, and when Henry finds out, he joins in on the phone sex instead of being horrified and offended.
The naughty babysitter
They leave their kids with a babysitter who is also Liz's daughter, Jennifer. Jennifer isn't old enough to drive herself back and forth, so Paul has to drive her home. When he asks more about Elaine and Liz's friendship, Jennifer responds with vulgar, abrasive language. Jennifer is ironic, because she is young and "uninformed" about so much, but she already knows the biggest secret of all, as far as Paul is concerned. Her naughty attitude and language are also ironic, given her age and relationship to Paul.
The happy ending
In the end the marriage is saved, because the couple agrees that the normal American suburban life was not healthy or fun, and they spontaneously burn down their own house. After this crazy ironic scene comes another crazy ironic scene: at the motel room that evening, as the kids fall asleep, the couple talks and they begin to feel truly connected again. This is ironic because there is absolutely no indication that their story would have a happy ending, and then it does, in a way.
