Well, happily, the subject of wit and irony in Austen's great novel is so rich that we can only start the subject here. In general, there are several kinds of irony and a wide variety of wit. The ironies are situational (when Lady Catherine de Bourgh approaches Lizzy about marrying Darcy, for example, she helps create the very thing she wants to avoid), social, and linguistic.
The wit is, as noted, more varied. There is the wit displayed by the characters, as when Lizzy (ah, Lizzy!) says the following to Jane: "Well, he certainly is very agreeable, and I give you leave to like him. You have liked many a stupider person.'' (How wonderfully silly!) There is also wit displayed by the author, as in the great novel's great first line (yes, I love this book): "It is a truth universally acknowledged, that a single man in possession of a good fortune must be in want of a wife."
To state such a thing as the novel's first line establishes the tone forever.