1. A situation where the result is a complete reverse (and practical mockery) of what was expected
2. A word heavy misused and abused in conversation today, mostly by people who think that using the word in any way will automatically make them seem intelligent. The word is usually misused to exactly mean "coincidental" or "tragic", when again it doesn't mean this (see #1)
If the heavy metal band Anthrax dies of anthrax, that would be poetically tragic, but not ironic. If a diabetic crosses the street and is run over by a truck carrying insulin, then it's ironic.
"Isn't it ironic that the pop song 'Ironic' contains absolutely no examples of irony?"
"One issue of Mad Magazine showed Alfred E. Neuman face-down in the desert, crushed to death by a parachuted crate of first aid supplies. Now THAT was ironic!"