In Pride and Prejudice, wit is the allure of the narratorial voice and irony is the lens through which the errors, transgressions, foibles and follies of the characters are seen and the brush with which their lives are drawn. Wit in the 18th and 19th centuries (even though Jane Austen's first and subsequent novels were published in the 19th century, they were written in the 18th century) encompassed a larger idea than humorousness. Wit entails a great intelligence, one that can express ordinary observations with an original insight born of mental acuity and one that see the differences between what is expected and what is actual and can find and convey the ironic mirth in the comparison of the two.

Jane Austen's gift is for witty irony. She has the ability to drawn on her small piece of "ivory" true to life characters, complete with all their flaws, who are beloved to Austen's readers precisely because they are seen truly with their good and their bad...with certain exceptions. However, even the unlovable exceptions like Mr. Collins and Lady De Bourgh are revealed under the light of irony with such a delicate hand that they aren't wholly despised.

The opening line of Pride and Prejudice is the most oft quoted example of Austen's witty irony. She says: "It is a truth universally acknowledged that a single man in possession of a good fortune must be in want of a wife." The wit is most evident in the words: "It is a truth universally acknowledged...." For one thing, universal truths are usually very large ideas that are very serious and very important. To think of rich bachelors "wanting" wives in that context is always worth a chuckle. The irony is most evident in the idea that wealthy single men are in search of wives. In truth, it is probably universally acknowledged that wealthy single men are the last ones to go looking for wives.

Wit in Pride and Prejudice provides intelligent glimpses into human nature and life choices through original juxtaposition of ideas and astute observations. Irony in Pride and Prejudice presents human truths and realities through situations, beliefs and narratorial observations that shows an opposing reality to what is expected to more clearly enlighten what is actual.

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.

This novel is, by far, Jane Austen most ironic and wittiest writings - which is probably why it is her most well known.  She uses ironic statements throughout her narration to comment on social behaviors in her world.  The novel begins with one such statement:

"It is a truth universally acknowledged that a single man in possession of a good fortune must be in want of a wife."

The generalization and the assumption in this sentence - that all people know that a single rich man must want a wife - is humorous and sarcastic.  She is attacking the attitude of society that sees this man as a possession to be grasped by the most enterprising mothers and daughters. 

It is not only in the narration that Austen uses wit.  Her most admirable characters in the novel are those that are themselves witty.  Mr. Bennet, for example, is described as thus:

Mr. Bennet was so odd a mixture of quick tarts, sarcastic humor, reserve, and caprice, ....

While he has his faults, readers are meant to admire this sarcastic man - and thus, to admire the way in which he shuns the standards of society.

Elizabeth Bennet, the hero, is also a witty character that uses irony to her advantage:

she had a lively, playful disposition, which delighted in anything ridiculous.

I don't have time to go into the situational irony that occurs in the novel, but suffice it to say that wit and irony are both plot developers and character developers in this story.

How can one discuss Austen's use of humour and irony in Pride and Prejudice?

The best way to discuss Austen's use of humor and irony would be to name a couple of different examples. Although, it should be noted that when looking at Austen, it is often difficult to treat humor and irony separately since for the most part she uses irony to create her humor.

We can certainly see many examples of Austen's humor in the character Elizabeth due to her quick tongue and ready wit. One of the best examples of Elizabeth's wit takes place the time she stays at Netherfield to look after her sick sister Jane. One evening while Miss Bingley is playing lively Scottish music on the piano, Darcy turns to Elizabeth and asks if she feels like dancing a reel. Elizabeth gives a very witty reply with the intention of making Darcy look like a fool in his pride when she responds by saying he only asked her that so he could insult her taste and, therefore, will not permit him to do so, as we see in her lines:

You wanted me, I know, to say "Yes," that you might have the pleasure of despising my taste; but I always delight in overthrowing those kind of schemes ... I have therefore made up my mind to tell you that I do not want to dance a reel at all--and now despise me if you dare. (Ch. 10)

Not only are these lines witty, they can also be seen as an example of verbal irony. Verbal irony is when the speaker says the exact opposite of what he/she means, often sarcastically. These lines are most definitely spoken with sarcasm. Not only that, it's probably untrue that Elizabeth does not want to dance a reel; she would probably very much enjoy dancing a reel, but is only saying what she says to insult Darcy.

Another example of Austen using verbal irony to create humor can be seen in the very opening lines of the book:

It is a truth universally acknowledged, that a single man in possession of a good fortune must be in want of a wife. (Ch. 1)

This example of verbal irony applies to the narrator as it is the narrator who is speaking sarcastically. The irony is that it is only mothers and daughters who believe a wealthy man "must be in want of a wife." The men themselves probably give it little consideration, preferring instead to enjoy their freedom and their wealth by themselves. Therefore, the opening line does not state a "universally acknowledged truth" at all, but rather a notion held by mothers and daughters. Hence, the opening line says the exact opposite of what the author/narrator really means.

How can you comment on the use of irony in Pride and Prejudice?

Irony, the contrast between the actual reality of something and the appearance or expectation of something, is one of the main stylistic components of Pride and Prejudice.
The very first statement of the novel, “It is a truth universally acknowledged, that a single man in possession of a good fortune must be in want of a wife,” sets the tone for a work of irony. In reality, it is mostly the women in the book who are chasing after men in order to secure a fortune. As the title suggests, the novel explores how people’s pride and prejudices get in the way of their feelings and actions. Elizabeth and Darcy begin the novel hating one another. Each thinks the other is stuck up and not worthy of their attention. And yet, they overcome these prejudices and much hurt pride and come to love one another. Austen uses irony in her works in order to expose the true nature of her characters and allow them to realize their prejudices, overcome their pride, and grow as people.