Dramatic irony is when the audience knows something which part/ or all of the cast does not know.

For example in Macbeth “Act 1 Scene 4”, Macbeth was saying “aside” how he had to become king. “The prince of Cumberland! That is a step on which I must fall down, or else o’erleap, for in my way it lies.”

This is a clear indication of Macbeths venomous attempt to become king.