The Irony of Keira's Healing
Keira has leukemia when she's first introduced, but it is cured by the infection of the microorganisms. Her infection is her healing oddly enough. In the end she owes her life to the aliens, which leads to a particularly unsettling pacifistic attitude which implies that she will now serve the aliens however they see fit.
The Irony of Eli's Resistance
Eli is motivated by the survival of his species to resist the urges of the symbiote to propagate. He does his best to ensure the infection will remain controlled and localized. Despite his hard work, however, the entire scheme is brought down by Blake's own urge to survive. Blake legitimately thinks he needs to leave this place in order to live. He doesn't know about the infection until it's too late, and he's spread it irreversibly. In the end, Eli's good intentions were wasted by someone with similar motivations and less information.
The Irony of Twins
Anytime twins appear in a narrative it's a safe bet the author is inviting comparison. In the case of Keira and Rane the conversation is about survival. Since she has cancer, Keira is doomed from the start. She is surprisingly healed of her leukemia by the infection. In contrast, Rane is perfectly healthy. The infection has the desired affect on her which eventually gets her killed by a group of homeless people.
The Irony of Extraterrestrial Life
When people think about extraterrestrial life, the normal association is aliens coming to Earth by their own will. What happens when humans are responsible for finding them and bringing them back? The Clay Ark mission wasn't supposed to find life, but they did. Unintentionally they sealed the fate of their species, initiating their own alien invasion.
The Irony of Death
In contrast to normal associations with death, in this book it seems to be a welcome solution. Eli's victims who die are free from his obsessive impulses and no longer pose threats to the human race. Those unfortunate women who are not lucky enough to die end up giving birth to young clayarks. Their own survival was the worst possible outcome for them as they become baby-producing slaves who must watch the gradual destruction of their species.
