Perhaps the grandest irony in the text. It seems all along that Hermes will have to return to Earth, and that any rocket launch from Earth to Mars will need to carry supplies directly to Watney. The Purnell Maneuver turns these expectations on their head, allowing for Hermes to turn around and rescue Mark, and preventing what would have been a higher-risk full-supply mission to Mars—one with minimal chances of success.
The Hermes crew and Watney's survival (Dramatic Irony)
NASA purposefully withholds information about Watney's survival from the crew of Hermes, leading to a situation of dramatic irony, where the reader knows something that certain characters don't. Kapoor and others fear that, if the Hermes crew knows that Watney is alive, they might not be able to carry out their mission effectively. Kapoor also fears that the crew will want to turn back and help Watney—which, of course, is exactly what Hermes winds up doing.
Watney's sarcasm on the videolog (Verbal Irony)
Watney's verbal irony often has him stating conclusions that are either obviously true or obviously false. In the first case, the obviously true is used for "deadpan" comic effect, since Watney understands just how difficult survival on Mars can be. In the latter case, Watney uses robust verbal irony to indicate that he doesn't actually think, for example, that green Martians will sneak into the Hab and abduct him (although he likes to refer to "Martians" playfully throughout the novel). Watney's sarcasm on Mars is continuous with his behavior on Earth, and helps him remain optimistic during an otherwise difficult mission.
Watney asking about the Iris (Dramatic Irony)
A piercing instance of dramatic irony, which relies on the comms lag between Earth and Mars. Watney asks Kapoor if the Iris was a successful launch after Kapoor, and everyone else on Earth, realizes that the rocket has blown up on lift-off. Watney will soon learn this to be the case, but his question shows just how lonely Watney is on Mars, and how dependent he is on Earthly solutions to get him adequate food and find him a rescue vehicle.
The Martian Questions and Answers
The Question and Answer section for The Martian is a great
resource to ask questions, find answers, and discuss the novel.