Rilke is magnificent on irony - but i'm not completely sure it fits in with our modern take on the word.
"Irony: Don't let yourself be controlled by it, especially during uncreative moments. When you are fully creative, try to use it, as one more way to take hold of life. Used purely, it too is pure, and one needn't be ashamed of it; but if you feel yourself becoming too familiar with it, if you are afraid of this growing familiarity, then turn to great and serious objects, in front of which it becomes small and helpless. Search into the depths of Things: there, irony never descends - and when you arrive at the edge of greatness, find out whether this way of perceiving the world arises from a necessity of your being. For under the influence of serious Things it will either fall away from you (if it is something accidental), or else (if it is really innate and belongs to you) it will grow strong, and become a serious tool and take its place among the instruments which you can form your art with."
i'm sure there is more in the book itself
http://www.sfgoth.com/~immanis/rilke/letter2.html
there is also a good essay on irony - its current use/overuse and the confusion which it engenders (not least because no-one can seem to agree on its definition and ,ironically, everyone seems to use the word wrongly now - at the back of the paperback edition of Dave Eggers "A Heartbreaking Work of Staggering genius". (MacSweeneys haters, please only criticise this essay if you have actually read it, thanks). It's well worth checking out but i doubt it's available online.