There's a great description of the nature of irony in this article by Anthony Esolen, entitled "Emptying Ourselves of What We Think We Know." The whole article is interesting, but click on page three just for the treatment of irony.
Esolen gives several divergent examples of irony, and manages to boil down the essence as something beautifully oriented to truth and reality, rather than the common conception of irony as simply "saying one thing but meaning another."
Irony arises, rather, from the ignorance of unseen or unexpected order (or, as it may happen, disorder), from the failure to note subtleties, or from seeing subtleties that are not there, especially when the ignorance and the failure are highlighted before observers in a better position to see the truth.
I really learned a lot from the five examples he gives on page 4, along with the subsequent elucidations for each one. Esolen very brilliantly and clearly manages to show irony's versatility: one example uses irony to teach theological subtlety while another points to the laughability of blind pride; one highlights a common sense of justice, while the last efficiently portrays a complex of relationships, intentions, and levels of ignorance that are dizzying when he explains it all out.
Irony provides humans with a way to communicate certain realities in a way that really does them justice: sometimes that feeling of unexpectedness shows just how amazing a truth really is, sometimes communication needs to play on the audience's sense of morality or poetry to drive home a point's real significance. Plus, when we have had to think a bit to figure something out, it stays longer in the brain than. So irony is a higher level of communication than just-another-declarative-sentence, and as Esolen point out, one that applies to communication with and without words (verbal and dramatic).