Knowledge (Verbal Irony)
The book's name itself is a wonderful joke on how truly impossible it really is to know everything, or even to know a little bit about everything. Imagine how voluminous a work like that would be just for this planet. A Hitchhiker's Guide to the entire galaxy would no doubt be both an impossible endeavor and a pointless one, because, as Dent learns through the novel, the journey through life (vis-a-vis his journey through the universe) is random and often senseless.
Murphy's Law (Dramatic Irony)
Murphy's Law is not explicitly mentioned in the novel, but it does help describe how Dent's experience of the universe unfolds. Murphy's Law is an adage that states, "Anything that can go wrong, will go wrong, probably in the worst way possible." The irony of the story's treatment of that truism is that, while Dent is often subject to the most frustrating, ironic troubles, he survives.
The tone of the novel itself might offer some sort of advice when dealing with the infuriating circumstances of life (such as the earth's destruction, for example), which is to adopt a sense of humor that can shrug it off. Things go terribly wrong for Dent, and he almost gets his brain removed from his body; but in the end, he ends up at a diner, which isn't so bad.
The Meaning of Life (Situational Irony)
One of the most ironic elements of the novel is that the meaning of life is "42." Well, at least, that's the correct answer to the correct existential question, but as of yet, the supercomputer Deep Thought has not been able to figure out the correct existential question. This is a charming, left-handed approach to the problem of existentialism, which is likely why this novel remains such a literary classic.
So, if 42 is the answer to the question of life, then here are some of the questions that cannot be the right existential question: Is there a God? What is the meaning of the human experience? Essence or Existence? Chicken or Egg?
If the answer is the number 42, the question can't be expository or philosophical. It has to be a quantifiable question. Many interpretations are available for understanding the novel's suggestion here, but, ironically, it seems that the point is not understanding the philosophy at all—the truth is out of our comprehension.
This irony is further explored in the attempt to harvest Dent's brain, because even though he doesn't cognitively know the 'question of life', he might have accidentally learned it along the way. If he might have learned it accidentally, then maybe the suggestion is that we might also know it if we're not looking for it and if we try not to think about it.
Irony of Technology (Situational Irony)
First, this story contains a depressed robot, which is fantastic as far as irony is concerned. As of yet, artificial intelligence is not capable of emotional experience (and many believe that machines could never develop soul), but the irony isn't necessarily that in the novel a robot has a soul (in fact, that's a well-explored question in the science-fiction genre): the irony is that the robot is sad.
Another irony concerning technology is the supercomputer Deep Thought. The first irony here is that the computer is concerned with the meaning of life, which is strange for a computer to care about, at least at face value. The second irony is that the earthlings are an extension of technology, and not the other way around.
