Irony in literature can be situational, dramatic or verbal. Irony is the occurrence of a statement or situation that opposes what is expected. For instance, Elizabeth Bennet visiting Pemberley when Darcy rides up is situational irony. Jane Austen saying that it is well known that wealthy bachelors are in need of wives is verbal irony. If the reader knew the truth about Wickham before Elizabeth did (we of course do not), this would be dramatic irony in which the reader knows more than the characters.
The main theme of Pride and Prejudice is denoted by the title and is...pride and prejudice. These are internal states of being, attitudes and beliefs. Pride and prejudice are related to appearance and reality, however appearances and reality are external elements to be observed and perceived and are not internal states, beliefs or attitudes. Opposition between appearance and reality can be created by many different states of being, attitudes and beliefs.
Having said this, Austen uses irony to shape the adjacent theme of appearance versus reality through narratorial comment, character perception and ironic situations. Austen uses narratorial comment for stating verbal ironies that point out the idea of appearances being in opposition to reality. For instance, her opening line indicates that society in small towns sees newly introduced bachelors as fitting suitors for unwed daughters, whereas in reality said bachelors may have rather different objectives in mind altogether.
Austen uses character perception to build the theme of appearance versus reality. A prime example is Elizabeth's belief in the appearance of Wickham's veracity, whereas time reveals a shockingly different reality relating to Wickham. Austen employs ironic situations for building this theme as in the situation in which Mr. Bennet permits Lydia to go to Brighton convinced that she will be kept safe and that the trip will rid her of some of her immaturity. Of course, the opposite of Mr. Bennet's expectations occurs.