The irony of innocence
The opening of the novel is ironic, because it establishes Dorrigo's character as an innocent person trapped in confusing situations, and in the background, he knows he's going to Japan soon. But if he could know beforehand what Japan would really be like for Dorrigo, certainly his girl drama would seem completely unimportant, and he certainly would not have wanted to go.
The ironic engagement
Ironically, Dorrigo is engaged through his imprisonment in the POW camp. This engagement is ironic because he doesn't love the girl, and because he did not propose to her (she simply misunderstood him), and because he doesn't actually want to marry Ella at all. Truly, he is not interested in her. Perhaps most ironically of all, by a series of misunderstandings and misinformation, he ends up marrying her anyway.
The ironic "death" of Amy
The main reason Amy's death is ironic is because, although Dorrigo believes the story he hears about her dying in an explosion, Amy is in fact not dead at all. Someone wrongly identified one of the blast victims as Amy, but in reality, Amy survived. This is even more ironic because Dorrigo doesn't find out until after he has already married Ella.
The irony of Darky's sacrifice
One of the ways Darky's sacrifice is ironic is because Dorrigo makes the decision to sacrifice Darky. This unusual role as Darky's decision-maker haunts Dorrigo for a long time, but Darky did not die in vain; his death helped many prisoners survive (which is itself ironic). Also there is dramatic irony, because the reader (and Dorrigo himself) do not know that secretly, Darky is Dorrigo's own nephew.
The irony of Dorrigo's fate
After a noble and difficult life, what could be more ironic than dying at the hands of a stupid teenager who drinks and drives? In other words, Dorrigo's death is ironic, because it seems unfair, considering the grand struggle of his life. Also, it's a reminder that drinking and driving is a horrible decision to make, and that although the teenager wasn't expecting to kill anyone that night, he killed a literal war hero (situational irony).
