That wasn't irony. THIS is irony (i.e. that the person who called out the other guy for not understanding irony, happens to not actually know the difference between irony and sarcasm himself) :P
ewjordan's Law of Irony: in any Internet discussion of sufficient length where the word "irony" is used, someone will call out the poster for using it incorrectly. This is true whether or not the original usage was proper or not, and with high probability the original complaint will be followed up by a comment on the irony of the critic not understanding the definition of the word "irony". Bonus points are awarded based on how many levels this back and forth continues.
Alanis Morissette will be brought up in at least 50% of these cases, though the precise frequency depends inversely on the level of sophistication of the forum members.
More specifically:
Some psycholinguistic theorists (e.g., Gibbs, 2000) suggest that sarcasm ("Great idea!", "I hear they do fine work."), hyperbole ("That's the best idea I have heard in years!"), understatement ("Sure, what the hell, it's only cancer..."), rhetorical questions ("What, does your spirit have cancer?"), double entendre ("I'll bet if you do that, you'll be communing with spirits in no time...") and jocularity ("Get them to fix your bad back while you're at it.") should all be considered forms of verbal irony.
Alanis Morissette will be brought up in at least 50% of these cases, though the precise frequency depends inversely on the level of sophistication of the forum members.