The thoughtful animal
The plot is ironic, because it supposes that while birds are flying around, perhaps they, like humans, are thinking about time and meaning, hoping for something. The irony of the image is that, although it seems out of place to consider a bird with such deep ideas, it could be said that human beings are equally absurd, because humans are animals too, and yet, we do wonder about the meaning of life.
The ironic progress
Although technology improves life on the ground for the people, the heron sees the progress of man from his bird's eye view, and he can't help but notice the pollution caused by progress, the chimney fires, and below, he notices that life is very complex and difficult, with people in every rung of the social hierarchy. He sees that time doesn't always make things better, as progress implies.
The ironic ineffability of meaning
Although meaning is thoroughly the point of this novel, the bird doesn't ever feel like he finds it. The reader can find it though, because the book is clearly arguing for the ineffability of life's meaning. There is meaning, but not in only observing one's own point of view, perhaps. For the reader, the heron is meaningful, but the heron doesn't believe that about itself. It's a melancholic bird.
The irony of relentless searching
He doesn't know what he's even looking for, the bird. In some ways, he's looking for nothing, and in other ways, he wants everything. He wants mastery of existence and godlike understanding of the natural world, but this only leaves him relentlessly pursuing something he can't define and has no real chance of discovering. The relentlessness of the journey is the novel's thematic punch, because the bird has no real reason to continue searching, but he does.
The irony of circles
The bird notices occasionally that things go in circles. He thinks a lot about wheels and human machinery, because many of the most obvious human inventions are wheels, like mills and factories with gears and wheels. The heron itself travels in instinctual circles, and over the course of the year, he moves around and then returns back to where he started. Although the story is linear, it is circular too, and the heron doesn't quite see that whole circle. He sees the present moment.
