' . . . as long as the state permits itself to interfere in the affairs of literature, literature has the right to interfere with the affairs of state.'-- Joseph Brodsky
Dramatic Irony is called that for a reason. It all began as a technical device when Sophocles decided to have old dim-witted Oedipus slog after that guy who murdered his father and slept with his mother. The Irony: We suspect he's the guy who did it.
Sophocles plants that awareness up front. So we know more than Oedipus and thus wouldn't do what he's doing. The result of this Dramatic Irony is a marked increase in our tension and concern. We're sitting there yelling -- in our minds anyway -- 'Don't do that, you knuckle-head, don't you see where this is leading?!'
Another way to look at this: A character who sees the yellow caution signs proclaiming Personal Calamity Ahead, but does what they're going to do anyway, may just be an idiot. And we could care less. [Unless you're really clever about it as Arthur Miller was in A VIEW FROM THE BRIDGE.] But if this same character forges ahead out of ignorance of facts that would be difficult for them to
A sort-of Rule: By letting us sense the truth of a situation in your play -- rather than telling us directly -- we'll buy into the Dramatic Irony of your script without sensing the creaking of a mere theatrical device.But . . .
When you're establishing the conditions for Dramatic Irony, you can skate very close to the edge of Spoken Subtext. So, Sophocles has a minor character tell us Oedipus is the murderer he seeks. But it's done in a way that allows us to believe this may not be the truth. He tells us, but at the same time he doesn't tell us. And that's the key to clever use of Dramatic Irony.
http://www.vcu.edu/arts/playwriting/