The irony of knowledge
The irony of knowledge is paradoxical beyond belief, literally. For Locke, knowledge is not what it seems like and not what we'd expect it to be. This is especially clear in Book IV, when Locke analyzes the way humans come to their beliefs, showing all the room for error, until the entire idea of objectivity is called into question.
The irony of nature and nurture
Nature and nurture is an ancient philosophical problem. Basically, does human knowledge originate from learning from experience, or can knowledge sometimes be inherited? Locke's answer was ironic—no nothing can be inherited. This was especially ironic during his time, although the popularization of the tabula rasa idea has made his claims seem less aggressive as they once were.
The irony of language
Animals don't have the same language processes of humans, so language is a fundamental aspect of any philosophical discussion of epistemology. The irony is that language processes might actually have some sort of influence on the type of knowledge that humans acquire, instead of just being a method for communication.
The irony of objectivity and subjectivity
By acknowledging that each object has inherent properties, Locke asserts that a kind of objectivity exists in knowledge, and in this case, subjectivity would reference the properties of an object in relationship to a person, as in 'Apples are tasty.' That's a purely subjective belief. Ironically, Locke goes even further, turning the microscope back onto knowledge itself. What can be said about the literal truth of a knowledge claim? For Locke, the answer to that question is 'pretty much nothing.' We're making assumptions, according to these ideas, every time we choose to believe our understanding of something is absolutely true.
The irony of innate knowledge
During Locke's time, his argument against innate knowledge was controversial. Many people believed the other view, that each human is born with a long list of 'human' ideas in their head that they build upon through experience. Locke says that a child's mind is essentially empty, and that they can fill it in, but even the way they fill it in could be easily scrutinized.
