Austen employs dramatic irony in Northanger Abbey to highlight Catherine's ignorance about Isabella. She has no idea that Isabella makes friends with her simply to develop a relationship with her brother, but the reader knows about it. In Volume 1, Chapter 6, Isabella says: "[...] there is nothing I would not do for those who are really my friends. I have no notion of loving people by halves, it is not my nature."
Isabella's descriptions of love and friendship, as well as her claims to love people wholly, are ironic because she only pretends to love people. Readers understand the disparity between her claims and her true character. Catherine, however, does not know Isabella's true intentions. This is an example of dramatic irony because the reader knows what is going on but the character in the story does not.
The function of dramatic irony in this case is to highlight Catherine's innocence and naivety. The reader already knows about Isabella's manipulative nature, and even the Tilney siblings catch onto the fact that Isabella is rather cunning. But Catherine herself remains ignorant of the extent to which she is being manipulated, ad this only makes it all the more clear that she is overly innocent and gullible.
