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.