The irony of the moral arguments for lobsters
This is the central irony of the collection, that the manner by which two differing viewpoints disagreed was actually unethical, even though they were arguing about ethics. The argument about the lobster concerns whether it is unethical to cook them alive, because they might be able to perceive suffering.
The irony of political discourse
This is probably not groundbreaking for anyone, but political discourse in America is about almost everything except actual policy. But Wallace's exposé of this irony is unique, drawing comparisons to religious believe and the nature of our perception of reality. Check the Mrs. Thompson essay and "Up, Simba."
The ironic relationship between Updike and his latest novels
This irony has the honor of being the only one that Wallace says emphatically, without the layer of artistry that usually conceals Wallace's points. Wallace just ends his essay about Updike by saying something to the effect of, 'Updike doesn't see that the reason he is unhappy isn't because he isn't getting laid; it's because he's an asshole.' The irony is that the thing that Updike explores to make him feel less alone is the very thing that makes him feel so alone: his unbridle misogyny and self-centeredness.
The irony of Kafka's humor
Wallace says that the reason his students don't get Kafka's humor is that they believe it is a thing to be gotten. This is a complicated assertion, but it seems to indicate that Wallace believes Kafka's humor is on a different level than the intellect. It's supposed to resonate within a person's very sense of self and the suffering that existence means for each person, but when you try to percieve it in the mind, it seems meaningless.
The irony of language and meaning
In the essay on American usage, Wallace shows how even though it is commonly thought that language proceeds meaning (that we use language to express meaning), it's probably the other way around, actually. It's in our view of language itself that we find our most intimate beliefs about the world. This essay beautifully articulates the difficulties of understanding reality. This is a special talent of Wallace's, because of his academic history with language theory and his studies of Wittgenstein.
