The ironic title
The novel suggests that the book ends with true restoration, but since the narrator is the protagonist, these claims about restoration could be simply a fever dream, brought on by plague. The character might be restored, if he survives his sickness, but by the time the novel is over, Restoration is what the character needs most.
The irony of Celia
Celia is an ironic wife for many reasons. First, she isn't even interested in Robert; the marriage is a sham. Also, Celia doesn't represent romance, love, acceptance, or anything that marriage seems to imply. She reminds Robert that he isn't good enough, and that he will never be truly powerful, and she rejects him. Or at least, that's how he chooses to interpret that relationship.
The capricious king
A king is supposed to be regal and moral, but this king raises a few eyebrows with his sense of justice. In either case, the standard for Robert is clear: loyalty to the king means a life of ease and luxury. But instead, Robert decides independence, becoming competitive with the king. By the end of the novel, this ironic relationship becomes nearly religious in effect, as Robert desperately wishes he had stayed on the king's good side.
The irony of synchronous illness
Modern medicine doesn't believe that illness is a punishment from God for poor behavior. But in the novel, that does seem to be the case, making the illness ironic. Robert's decline in mental health is also watched by his friend who works with mental health patients for a living. The implication is that perhaps Robert's simultaneous insanity and illness are the product of his own darkness and negativity.
The irony of forgiveness
Possibly in a fever dream, Robert sees a vision of the king restoring him to his full wealth and privilege. Even if it isn't real, even if he's making things up in his head, Robert still has a religious thirst for forgiveness and restoration. The irony is that Robert has spent his entire downfall cursing the king, but now at the end of his sanity, all he can dream about is whether he will be forgiven or not.
