How does the chapter title create dramatic irony?

Not at home is very straightforward and refers to the fact that Smaug is not in his lair. It is an example of dramatic irony because of Thorin's reaction when he tells the others, "Don't call my palace a nasty hole! You wait till it has been cleaned and redecorated." His rebuke reminds us that the dragon's lair wasn't always its home.... it was the home of Thorin's ancestors.