You could probably discuss a couple of different ways in which act 4, scene 5 is dramatic or contains dramatic moments. In terms of irony, the scene contains a heavy dosage of dramatic irony. This is because the audience knows one huge key fact that most of the people in the scene do not know. Juliet is not actually dead. Lord Capulet, Lady Capulet, the nurse, Paris, and the musicians all believe that Juliet has died. The audience, of course, knows that Juliet is not actually dead. She only appears to be dead because of the friar's magic potion.
Despite the fact that we know Juliet is alive, the scene is still dramatic and tense because there is a huge amount of information that we are still unsure of. Juliet has to arrive at the tomb, and Romeo needs to be there. There are a lot of moving parts to the plan still to be completed.
Then the final part of the scene actually extends our tension. All of the key characters exit the scene, and we are left to listen to the musicians talk about their music, their payment, and the death. A person might argue that this portion of the scene functions as comic relief, but I think it actually holds audience tension even longer. We are not rewarded with the story continuing into act 5. We have to wait for answers. It's like when a TV show brings audiences to a very tense and dramatic moment between the man and the woman only to break for a commercial. Our suspense isn't relieved by a funny commercial. It is heightened because we have to wait even longer for answers.