Dramatic irony hinges on the audience knowing something important that one or more characters don't know.  When  Romeo hears of Juliet's "death" and rushes back to Verona, the audience knows that a messenger is en route to tell him of Juliet's plan, but the ill-fated Romeo does not know.  When Romeo goes to the Capulet tomb to say good-bye to Juliet, he still doesn't know that she's actually going to wake up,  and he kills himself even as the horrified audience looks on.  Of course, Juliet wakes up, pieces together what has occurred, and kills herself for real this time.  The ending of the play, when the Capulets and Montagues join in their grief to end the feud and pledge to honor each other's children, comprises the final irony, for it was of course this feud which set into play the chain of events that ended with the deaths of Romeo and Juliet.