The irony of our biology
Malthus's central point is that we should use our unique human qualities to understand that we are still animals. This is ironic, because most of the time, science and reason are shown to make humans less like the other animals, but this essay does the opposite by focusing scientific attention to the fact that we're animals with instincts, and that we have population limitations, just like other species of animals. Malthus's argument is not that humans are different than animals, but rather that we are animals.
The irony of our dependence on natural resources
When Malthus wrote this famous essay, his readers might have been amused by his attention to the fact that we cost the earth something. Before he said it, not many people had paid such close attention to the fact that the planet is limited, and we depend on it for our biological life.
The irony of natural overpopulation
Malthus is not observing a phenomenon that occurs only to humans. Every animal is competing for the use of natural resources, but humans have the unique privilege of being able to predict future problems and fix them through science. So what makes us unique is not our problem, but our ability to work together toward a solution. The irony is that terms like "overpopulation" and "population density" sound like sociological problems, but they're actually the result of our animal instinct to make babies. We are in fact, not unique in this regard.
The irony of limited money
The idea of money is complicated and strange, and it's something that is unique to humans. Basically, the ironic aspect of fixed wages is that there is a conservation of resources, so if there are suddenly too many people and not enough money, then there could be a serious dilemma where there isn't enough currency to sustain the population.
The irony of proportions
This might be helpful in contextualizing Malthus's ideas: There is an irony between our perception and our reality, namely the ironic power of proportions. Although Malthus wrongly assumes that population growth is geometric (it's actually exponential), he is correct in his main idea that we humans tend not to understand the full potential of non-linear growth. In other words, our biological minds should be supplemented by real, scientific reasoning, because we aren't so good at understanding the macro-economics of our reality.
