there are different types of irony. Wikipedia atually has a very good right up on it .It's easier to say what it's not,which is the whole of Alanis Morisette's song "ironic". Its a phrase or situation which is used to make people believe/perceive the opposite of what is meant. Mac, Ive heard that one before. That only covers irony in conversation though, theres the type of irony that happens in plays and films where the viewers no more about the events than the characters eg actor killing someone because he wont have him commiting any crimes/murders when we know the the actor is actually an evil doer or will end up doing evil himself ( a bit like that program Dexter, or the death of Romeo and Juliet).
In true irony there should be some form of parrallel running between the two events.