Dramatic irony is used in the prologue of Romeo and Juliet presents facts to the audience that will be unknown the characters in the play. It is a means of creatingsuspense making the audience member anxious or excited about the upcoming plot.
In Act 2 scene 4, when Benvolio and Mercutio are talking to Romeo, Juliet's nurse arrives and as Benvolio and Mercutio are making fun of her being large and are disrespecting her, none of them know...
We know that Juliet is not dead and that Romeo is mistaken when he thinks so. It is even more ironic in that Romeo is supposed to know that she is alive--that is the whole point of the exercise.