The dramatic irony of Act 11 Scene 1, is that Romeo is totally in love with Juliet now. Mercutio and Benvolio, unaware of this new twist, try to find Romeo, who is hiding in the orchard. Romeo is thinking of nothing else but Juliet. His friends still think he is hung up on Rosaline, and try to get Romeo out of hiding, by teasing him about his "love" for Rosaline. Romeo wants nothing to do with it, so his friends leave.
We all know, that by this point, Romeo has fallen in love with Juliet, at first sight. He had gone to the ball to try to get close to Rosaline, but once he saw Juliet, he knew that he was not in love with Rosaline. She becomes a distant memory to him. The ironic part is that his friends don't know of the turn of events, so they are trying to give him hope about Rosaline. Romeo doesn't want to talk to his friends, he just wants to think about Juliet and how he is going to see her next. Romeo, thinking he came to the ball because of Rosaline, realizes that fate has brought him to this ball, so he could meet Juliet. Thus starting the tragic road of true love.
Dramatic irony is created when there is a discrepancy between what a character believes and what the audience knows to be true. In this scene, Mercutio and Benvolio search for Romeo and Mercutio teases him about his love for Rosaline. What Mercutio and Benvolio do not know is that, by this point in the story, Romeo has forgotten all about Rosaline and is completely fixated on Juliet.