The irony of the name change
A name is an unchanging word or sound that people use to reference someone. So it's inherently ironic anytime a person or character changes their name. This is a famous literary device, too, because names are such an obvious metaphor for one's identity or destiny. For Kate, that means something a little negative, because she changed her name from Daisy to Kate after being bullied in school. It's like she has chosen to permanently hide her floral qualities, harming her relationship to Violet in the process.
The irony of the affair
What could be better than two young, attractive couples hanging out and raising their families together? But for Kate, it ends up being a bad thing, because the wrong couples fall in together. When Jeremy and Kate's relationship is seen in all its strain and dysfunction, the next scene is Kate sitting on Hank's lap, purposefully betraying them all. Their families will surely suffer for this indiscretion, if Violet's prophecy is true.
The irony of prophecy
To see the future is a part of human psychology that no one can explain simply. Sometimes we experience the way things could happen or might happen in the future, and when those events turn out being true, the experience is a little startling and definitely ironic. But for Violet, that gets taken to a new level. She prophecies about catastrophes and missing people, and she goes on the news and tells everyone. Sometimes, she's right, like when her help solved an open case.
The irony of safety and protection
For Kate, her name is a signal of normalcy. Her marriage is a type of safety. Her friendships and lifestyle is unchanging and enjoyable, so she feels comfortable and safe when the novel begins. Violet, on the other hand, lives more freely, taking more risks, exploring her self and her potential. Violet believes that she is protected by higher parts of her consciousness, like Guardian, her personal angel by whom she prophesies. In the end, Violet's more chaotic life proves more stable, because she is not averse to change and chaos, but her sister is. In the end, the issue of safety is situationally ironic, because the sister that craved safety and order ended up bringing herself an untold future of chaos and suffering.
The irony of tragic downfall
This has already been touched upon, but it deserves its own entry: When Kate falls into her affair with Hank, she is alone and upset because her husband didn't believe the prophecy about the earthquakes. This is highly ironic because it means that Kate thinks she understands the prophecy when she is about to blindly invoke her fate. The implication is simple: Kate's affair with Hank is the catastrophic earthquake Violet predicted. The prophecy needed only to be interpreted correctly.
