Dramatic irony is when the spectator or reader is given information that one or more characters are not aware of. Therefore, a subsequent event is usually a surprise to the character, but the audience is ready for it.
At the very beginning of Part 3, Beatty is lecturing Montag on the perils of literature. Meanwhile, Faber continues talking to Montag via the radio device in his ear. The reader, Montag, and Faber are aware of this, but Beatty is not. Faber continually asks Montag if he can escape. Montag replies that he can't, "Because of the Hound!" Beatty assumes he is talking to him and agrees that the Hound is near. Finally, Montag tries to escape and Beatty hits him, sending the radio earpiece (the "green bullet") flying to the ground. Beatty discovers what we, Montag, and Faber had already known. This is called the resolution of dramatic irony. This is when the ignorant character discovers what the spectator was already aware of. Beatty says:
"Well--so there's more here than I thought. I saw you tilt your head, listening. First I thought you had a Seashell. But when you turned clever later, I wondered. We'll trace this and drop it on your friend."