Evan is Both Human and Other
"Which raised the troubling question: Why didn't he finish her that first day? When he heard the shots inside the convenience store and followed her back to the campsite, why didn't he finish her then, while she lay crying in the dark?" (138)
It is ironic that Evan has somehow become both human and Other. He is, presumably, the first Other that we know of to turn his back on his own kind in order to help the humans instead. It is also ironic that he has such strong feelings for Cassie, since as an Other who helped orchestrate the destruction of humanity, one would think that he feels hatred, or even nothing, for humans.
Sammy Doesn't Want His Bear
"'I, um, I thought you might want this back.' He frowns and shakes his head." (413)
It is ironic that Sammy doesn't want his teddy bear back when Cassie offers it to him, after finally being reunited. Cassie has spent the entire book carrying the bear around, talking to it, and using it as her beacon of hope. She has become attached to the bear and Evan even mistakes it as her own at first. However, Sammy has no interest in the bear anymore. He missed it the first few days he was at Camp Haven, but his time and training have had an effect on him. Sammy is overjoyed to see his sister instead.
Reznick as the Sniper
"Reznick. We both hear it. And it nails down our feet. It stops our breath. It freezes our thoughts." (312)
"'Spend months training us so he can kill us,' I say." (313)
It is ironic that Reznick is the sniper who killed a member of the squad, since he was the one who trained and drilled the squad for months. They were the first squad to graduate and should have been his pride and joy, yet he, an Other, has no problem destroying what he worked months to create. He even refers to them as "maggots" (312).
Cuticle Care
"He [Mitchell] also chewed his fingernails. That was the second thing I noticed. Cuticle care cam tell you a lot about a person." (22)
"Okay, don't panic. It's been a few months since he did any farm work. And you know how nice his [Evan's] cuticles are." (272)
It is ironic that in the face of so much danger and the amount of untrustworthy people that she encounters, Cassie values cuticle care. She values it very highly, mentioning it several times and describing how she appreciates Evan's nice cuticles. It's an ironic and minuscule thing to focus on when humanity is about to go extinct.
