The irony of Jim's job as warden
Jim's authority over nature is Edenic. Just like Adam, Jim's job was to be in charge of a garden, an estuary to be more specific, and just like Adam, Jim was required to leave that garden and by leaving that garden, he also is required to die. But where Adam was kicked out for sinning, Jim was kicked out by fate, as if the universe wanted a clear-minded, masterful person to experience warfare and give a report about it. His job as warden is coupled with a responsibility to leave the garden behind. This is highly ironic.
The irony of Jim's violence
When Jim practices violence, it goes against his instincts. He finds that he is able to use violence to protect himself, and in fact, he learns that unless he is willing to become violent, he will die, so the irony of violence is simply that for a soldier in a war, to be willing to kill might actually be the correct response, since the stakes are so high.
The irony of Australian participation in WWI
When Jim joined the war, he did so because Australia was politically joined to Europe, but that's ironic, because Australia is actually pretty far from the Western Front. That means that Jim traveled across the world to fight in a war, because he still felt European.
The irony of technological progress
The irony of progress is that inherently, better tools sound like a good thing. Mechanized processes allow for more efficiency, which means more trade, which means faster technological growth. The irony is that, actually, technological progress is intertwined with warfare, and the industrialized world is also the world of machine guns and mortar strikes and weapons of inconceivable power. Historically speaking, this was the case in both World Wars, and a theme, it's shown with absolute precision in the novel. Progress is not a good thing for the soldiers who are being shot at with machines.
The irony of human life and meaning
Jim's character starts as an ideal representation of a meaningful life. He follows his bliss and finds a career in the center of his passion as a naturalist. He loves nature, and he loves the variety and striking beauty of every kind of bird. That's like a metaphor, because it tells us that Jim is the kind of person who can find meaning and thrive on beauty. And yet at the end of his story, his death represents the futility of human life, and the horrors of nature. It seems that no matter how essential one's life might be, no one is immune to death or to the horrors of human evil.
