Resist the impulse to refer to things that are merely coincidental as ironic.
Irony is a complicated notion with an even more complicated history. Those with scholarly inclinations should check out Norman Knox's important book, The Word "Irony" and Its Context, 1500-1755 (Durham: Duke Univ. Press, 1961). Those without such inclinations should at least be aware that things are messy, and that precision is useful.
Irony usually refers to some kind of gap between what's said and what's meant. The simplest form is verbal irony, which in its crudest form is sarcasm. If I start praising the infinitely enlightened administrators who run my university, complimenting them for their inexhaustible wisdom and their clarity of expression, most people will recognize that I'm indulging in verbal irony. (The aforementioned bureaucrats won't, but they're beyond hope.)
Other kinds of irony are more complicated. Dramatic irony is when a speaker isn't aware of the true meaning of what he or she is saying (as when Sophocles' Oedipus vows he wants to punish the sinner who brought the plague on Thebes, unaware that he's talking about himself). Socratic irony describes Socrates' disingenuous pose of ignorance. There are also situational irony, structural irony, and romantic irony. Cosmic irony is perhaps the closest to what most people think of as irony: it's when God or fate seems to be manipulating events so as to inspire false hopes, which are inevitably dashed. Thomas Hardy's novels are filled with it. For all of these kinds of irony, the word ironic is just dandy.
But you shouldn't use it merely to suggest something coincidental or contrary to expectation. "He thought the plan would make him rich, but it turns out he lost all his money" — that's a pity, but it's not really ironic. Ditto "She spent years looking for her high school sweetheart, but he finally called her a week after she married someone else." Again, a damn shame for all involved — but not ironic.
A middle ground — sometimes accepted, sometimes not — is for unexpected things that, in the words of The American Heritage Dictionary, "suggest . . . particular lessons about human vanity or folly." Here's their example, which about three-quarters of their usage panel found kosher:
Ironically, even as the government was fulminating against American policy, American jeans and videocassettes were the hottest items in the stalls of the market.
The idea is that it wasn't merely an unintended consequence; it points up a serious problem of "human inconsistency."
Even though three-quarters of the panel found it acceptable, remember that one in four disagreed. For that reason, it's always safest to use words like coincidental, unexpected, improbable, or paradoxical when they're what you mean, and to reserve ironic for unambiguous cases of irony.
