Here's my stab at irony, discussed over at limetree.

Irony is about contradiction. There is a contradiction between two perspectives, and this contradiction is manifested as some sort of dissonance. It is not surprising that people use "irony" to refer simply to something unexpected or incongruous. Situational irony is simply the gap in perspective between expectations and results.

There is widespread tonal irony in contemporary literature that is characterized by a more diffuse or multidirectional clash in perspectives. This is known as postmodern irony and can take many forms. For example, I might not just say the opposite of what I mean (classic verbal irony) but rather

take a tone that is incongruous with the subject matter, treating it less seriously or more seriously than it deserves

pretend to invest affect in something that I don't actually care about that much, or adopt a "cool" affect in relation to something I do care about

take a "knowing" tone toward something I know nothing about: faux sophistication

take an "unknowing" tone toward something I do know about

adopt a tone that does not reveal what I really think about what I am saying, or mix several tones together

satirize something I really love, with the understanding that everyone understands that I love it and that the irony is directed against myself

parody "blankly," without any affect that reveals how I really feel about what I am parodying

parody so viciously that it becomes a parody of someone with a parodic attitude

use a tradition parodically, showing that I don't really respect its conventions, but adhering to these conventions enough so as to create doubt in the reader's mind as to what my true attitude is

The forms of insincerity are multiple. Nobody could list them all. Postmodern irony is metaliterary, in that it makes references to the literary discourses and traditions used. There is always some ambiguity about which direction the irony is pointed in. And it usually involves a particular tonal modulation that is more subtle than rank sarcasm.