The dramatic irony in act 5, scene 3 of Shakespeare's Romeo and Juliet arises from what the audience knows—which is a considerable amount of information—and the characters don't know about each other and about the situation in which they find themselves. Compared to the audience, the characters in the scene know almost nothing, which very much increases the irony in the scene and holds the audience in a heightened state of suspense and anticipation for the entire scene.
The audience knows that Romeo and Juliet are married, the circumstances of the marriage, and everything leading up to this scene, including the fact that Juliet isn't dead but simply in a deep sleep from having taken Friar Laurence's death-simulating sleeping potion.
Paris enters the churchyard where Juliet's tomb is located with the Page. Paris sends the Page off to warn him if anyone else comes into the churchyard. As he strews flowers outside Juliet's tomb, he is unaware that Romeo and Juliet are married, and that Juliet is not dead.
The Page whistles to warn Paris that someone is coming into the churchyard, and Romeo enters with Balthasar as Paris hides. Balthasar is unaware that Romeo and Juliet are married and that Juliet is not dead. Meanwhile, Romeo is unaware that Juliet is not dead.
Romeo give Balthasar a letter for Lord Montague, and Balthasar moves off to the side and falls asleep. Romeo then opens Juliet's tomb, and Paris confronts him, thinking that Romeo intends to defile Juliet's tomb. Romeo doesn't recognize Paris in the dark outside the tomb. Not knowing that he's speaking to Paris, he appeals to Paris not to fight him but doesn't tell him that he's married to Juliet. Paris presses the issue, they fight, and Paris is killed, still unaware that Romeo and Juliet are married, and that Juliet isn't dead.
With his dying breath, Paris asks Romeo to "lay me with Juliet," thinking that he was going to marry Juliet and that therefore he's entitled to lie next to her. Romeo then discovers that he's killed Paris. Romeo lays Paris in the tomb, although not with Juliet. Thinking that Juliet is dead, even though her lips and cheeks are still red and she looks "so fair," he kills himself with the potion he acquired from the Apothecary.
Friar Laurence enters, having no idea what has only recently transpired, thinking that he's going to enter the tomb and wait for Juliet to awaken, and then wait for Romeo to arrive. He doesn't know that Romeo didn't receive the letter that he sent telling him about the sleeping potion. Friar Laurence's heart sinks when he hears from Balthasar that Romeo is already in the tomb, and he's seriously alarmed when he sees the blood outside the tomb.
Friar Laurence enters the tomb just as Juliet wakes up. He hears some noise outside the tomb and urges Juliet to leave the tomb with him, which might have been more successful if he hadn't shown her Romeo and Paris's dead bodies and said that he was going to send her to a cloister of nuns. Seeing Romeo dead, Juliet first tries to take the poison that he took to kill himself, but the vial is empty. She takes his dagger and kills herself with it.
Within minutes, all of Verona is awake and has descended en masse on the churchyard—the Prince, the Chief of the Watch, Lord and Lady Capulet, Lord and Lady Montague, the Page, and everyone else the Page woke up in the middle of the night. Balthasar is found sleeping in the churchyard and brought forward, and the Friar is apprehended trying to run away from the scene.
Of all of those now assembled in the churchyard, only Friar Laurence can explain what's happened. What he tells everyone in the churchyard is everything that the audience already knows. Balthasar also gives the letter that he has from Romeo to the Prince, which supports Friar Laurence's explanation of events.
The extent of the irony in the scene is remarkable, as was Shakespeare's ability to resolve every aspect of Romeo and Juliet's star-crossed relationship in one extraordinary scene.