what dramatic irony is the storys plot driven by

The story's plot is driven by the fact that Fortunato does not realize that Montresor, the narrator, is going to kill him. This constitutes dramatic irony, because we know more than the character does, and this knowledge is responsible for the tension in the text. Montresor says the following:

It must be understood that neither by word nor deed had I given Fortunato cause to doubt my good will. I continued, as was my wont, to smile in his face, and he did not perceive that my smile now was at the thought of his immolation.

In other words, Montresor has maintained his usual demeanor toward Fortunato because he doesn't want his nemesis to have any inkling that Montresor is, indeed, plotting to take his life. Now, however, when Montresor smiles at Fortunato, it is because his plan to kill Fortunato pleases him, and he knows that Fortunato has absolutely no idea of what is coming. This irony persists throughout almost the entire story, until Montresor finally pushes Fortunato into the recess in the wall and chains him in.