Dramatic irony is when something happens that only one or a few characters on stage and the audience know about...all other characters are in the dark. In this soliloquy, Juliet is grieving that she is in love with Romeo, and he is a member of her family's arch enemy family. She is speaking on her balcony about how she loves him and how if he were any other name (from any other family) it would all be OK. She doesn't know it, but Romeo is in the bushes below listening to her every word. The audience knows this, too. Viola! Dramatic irony.