Dramatic irony is when the audience is aware of something that the characters do not know. Dramatic irony heightens the suspense for the audience and provides an additional layer of meaning.
This conversation, taken from Act IV, Scene 1 of Romeo and Juliet, is full of irony. Capulet and his wife do not know of Juliet's plan to fake her death and elope with Romeo. They are unaware that the "holy friar" they so admire is helping Juliet run off with the son of their enemy. When Lady Capulet says "there is time enough," she is not aware that in the morning she will find her daughter "dead" (and she will not know Juliet's death is a fake one).
Meanwhile, the audience is aware of Juliet's plan to marry Romeo despite the fact she is engaged to marry Paris. The audience is aware that the friar is helping them. They know he has provided Juliet with a potion that will cause her to sleep so deeply she will appear dead. Finally, the know that there is not enough time. When the go to church again, it will be for Juliet's funeral, not her wedding to Paris.
When the audience hears Capulet and his wife say these things, the effect is a strong one. The dramatic irony increases the suspense and adds to the tension about what will happen. Finally, the audience is left to wonder: how will the Capulets feel when they find their daughter dead? And how will they react when they learn "this reverend holy friar" played a part in this tragedy?
