The irony of attention
One of the main plot devices in the novel is the inversion of major and minor characters. Ironically, it's not Ernest Hemingway at the heart of the novel's intent. The novel is about Hadley, the hero of love who manages to continue loving Hemingway though he betrays and disappoints her, and though he never manages to return her love the way she might have desired. This asks the reader to pay attention to the people in the story who are not the "main character" types.
The irony of betrayal
Betrayal of someone's trust is ironic, since by definition it is shocking and disappointing. But Hemingway's betrayal of Hadley is ironic in its own regard—he asks her to move in with his mistress. This only highlights how self-involved the man has become.
The sad irony of depression
Depression is a terrible disease in which a person becomes tormented by their own emotional agony for long durations of time. One sad symptom of this issue is that it often promotes narcissism, which one might not expect. The case of Hemingway is clear: He mistreats Hadley because his mental health is compromised, and also because his morality has become corrupted by his narcissism. Hadley is also a victim of Hemingway's torment, as a scapegoat perhaps.
The Sherwood Anderson irony
It's a terrible thing to dishonor those who have helped one to succeed, but that's what Hemingway does to Sherwood Anderson in The Sun Also Rises. The pettiness of his artistic insult was even offensive enough that Gertrude Stein dissociated herself from Hemingway.
The irony of parenthood
For Hadley, a person is wired to give love, parenthood is meaningful, but for Ernest, it means the exact opposite of what it means for Hadley. For Hadley, it means hope and rebirth, but for Hemingway, it means death of the will and the end of selfish ambition. He resents the situation because he wants to be the center of attention.
