In Part Two, McMurphy is shocked to learn from Harding that Scanlon is the only patient besides him who is involuntarily committed to the mental hospital. There is situational irony at play in this exchange:
“You have more to lose than I do,” Harding says again. “I’m voluntary. I’m not committed.”
McMurphy doesn’t say a word. He’s got that same puzzled look on his face like there’s something isn’t right, something he can’t put his finger on. [Harding] swallows and says, “As a matter of fact, there are only a few men on the ward who are committed. Only Scanlon and—well, I guess some of the Chronics. And you. Not many commitments in the whole hospital. No, not many at all.”
McMurphy has been operating under the assumption that practically everyone has been involuntarily committed to the hospital. Conditions there are so abysmal that he never imagined anyone with a decent alternative choice would decide to live there. McMurphy, who was brought up on charges for a violent crime, had a choice between hard labor (probably building infrastructure like roads) or commitment to the mental hospital. He does not even consider himself mentally ill but would rather pretend to suffer from psychopathy than endure the sentence of hard labor. He has come to the hospital with the intent to disrupt the power dynamics. He struggles to wrap his head around the idea that the entire time, the rest of the men have been submitting willingly to Nurse Ratched's tyranny.
McMurphy is unsettled on his own account to learn that he is an outlier in his involuntary commitment. He has previously thought of himself as less in need of a mental hospital than everyone else there. Still, as an outsider, McMurphy is able to see the situational irony to which the men on the ward seem desensitized. This hospital, which is supposed to be helping them improve their mental health, is instead tearing them down and turning them into weaker-minded versions of themselves. The irony infuriates him and ultimately makes him double down on his mission to teach the other patients to take charge of their own lives.
In Part Three, Bromden wants to sign up for McMurphy's fishing expedition but is afraid to do so because of the dramatic irony he has maintained so long around his hearing. He is amused by the situational irony:
[I]t’d show I’d been hearing everything else that’d been said in confidence around me for the past ten years. And if the Big Nurse found out about that, that I’d heard all the scheming and treachery that had gone on when she didn’t think anybody was listening, she’d hunt me down with an electric saw, fix me where she knew I was deaf and dumb. Bad as I wanted to go, it still made me smile a little to think about it: I had to keep on acting deaf if I wanted to hear at all.
Bromden has faked his own deafness for years as an act of self-preservation. It allows him to fade into the background and collect ammunition against the hospital staff without anyone noticing. Now, this decision is backfiring on him. He realizes that in order to get what he wants (a spot on the fishing expedition) he would have to reveal his secret and make Nurse Ratched so angry that she would surgically debilitate him, making sure he could never hear or speak again. Ironically, a decision made out of self-preservation has put Bromden at great risk.
The fact that Bromden wants to go on the fishing expedition is testament to the change McMurphy has brought to the ward. Bromden has always disappeared into his hallucinations of fog, and the idea of leaving the hospital has only just started to occur to him. More than that though, Bromden's smile suggests that he is starting to adopt McMurphy's tools of resistance. When McMurphy is in a seemingly powerless position, he laughs at Nurse Ratched, at the other men, and at the situation he is in. Here, Bromden finds himself in a seemingly powerless position. He cannot have what he wants because declaring that he wants it would put him in danger. Whereas this feeling of powerlessness has previously led Bromden to hallucinate fog and to despair about the conditions on the ward, he now cracks a smile at the irony of his situation. This smile is not full-on laughter, but it does hint that Bromden is starting to use laughter and joy as tools to take control of his own experience on the ward.