In the book, an example of dramatic irony would be when the narrator reminds the readers of the solution in a riddle that the dwarves have forgotten: "They were too eager to trouble about the runes or the moon-letters, but tried without resting to discover where exactly in the smooth face of the rock the door was hidden." Suddenly, Bilbo remembers what the others have forgotten and is able to open the door. (This is in Chapter 11.) Hope this helped!