Northanger Abbey brims with verbal irony, as the narrator often says one thing while meaning another. For instance, in Volume 1, Chapter 14, the narrator states that imbecility enhances a woman's charms:
The advantages of natural folly in a beautiful girl have been already set forth by the capital pen of a sister author;—and to her treatment of the subject I will only add in justice to men, that though to the larger and more trifling part of the sex, imbecility in females is a great enhancement of their personal charms, there is a portion of them too reasonable and too well informed themselves to desire any thing more in woman than ignorance.
Here are two examples of verbal irony. First, there's the idea that "imbecility in females is a great enhancement of their personal charms." Second, there's the idea that smart men desire ignorance in a woman. Neither statement is true, of course, nor do they necessarily reflect the values of Northanger Abbey. What the narrator means is that intelligent women intimidate men and that men like to feel smarter than they actually are, especially in the company of women. In the context of this scene, Henry Tilney feels superior to Catherine because she seems ignorant. He also enjoys teaching her things about life and literature because she is so open, innocent, and encouraging. However, she is not stupid—she is simply naive. Her background and experiences differ greatly from those of the wealthy and well-educated Henry.
Here and elsewhere in the story, the narrator's humor seems hilariously insulting but remains harmless. It's often so exaggerated that it veers into sarcasm that imbues the text with a humorous tone. The characters also each have their own brand of verbal irony, and the narrator's voice brings them all together.