sarcasm is always a form of irony, but irony isn't necessarily sarcasm.

but the misuse that always amuses me is using "nauseous" for "nauseated"

nauseous means something is so disgusting that it make you sick - e.g., "Grampa's toenails are nauseous"

nauseated means to feel queasy- e.g., I saw Grampa's toenails, now I'm nauseated.

whenever someone tells me they are nauseous, I generally try not to giggle.