Irony

What is the difference between sarcasm and irony? I grew up thinking irony referred to those unfortunate or incongruous coincidences found in life and literature. Wasn't there irony in the Twilight Zone episode where the bookworm, the sole survivor of nuclear armageddon, comes upon a vast library, only to then lose his glasses? Only in the nonstandard sense. Irony really pertains to speakers and their expressions. Something is ironic when its literally meaning is different (or actually contradicts) its intended meaning. I'm embarassed to say that I didn't grasp this before Ethan Hawke's character in Reality Bites explained it.

So irony is just like sarcasm, then?

Well, I've known plenty of sarcastic jerks, but I've yet to meet an ironic jerk. I suppose that's because sarcasm, the "lowest form of wit", is a tone that happens to often involve irony. You can be ironic without being sarcastic, though. You can also be sarcastic without being ironic. Just repeat someone's unsatisfactory suggestion in a sneering, deriding tone. If the suggestion, as a prescription, has no propositional content, then neither does the sarcastic utterance. An ironic statement has to be propositional for there to be an incongruity (or does it?).