how does dramatic irony contribute to the action and plot of scene 5

Romeo approaches Juliet and touches her hand. They speak together in a sonnet, and Romeo eventually earns Juliet's permission for a kiss. However, before they can talk further, the Nurse calls Juliet to see her mother. After Juliet leaves, Romeo asks the Nurse her name, and is shocked to learn that his new object of desire is a Capulet.

As the party winds down, Juliet asks her Nurse about Romeo. When she learns about Romeo’s identity, she is heartbroken to find out that she has fallen in love with a "loathed enemy" Of course the audience knows, from the prologue, the fate of these star crossed lovers. The scene just adds to the dramatic tension that Shakespeare is carefully building in the play.