In Act IV, Scene I, Juliet arrives crying at Friar Lawrence's, where she finds Paris, who believes she is crying over Tybalt's death. In reality, Juliet is crying because Romeo, her love, caused Tybalt's death and must now be exiled away from her. In this same scene, Juliet states that she has not married Paris yet, but the audience knows she loves Romeo and does not intend to marry Paris.
Dramatic irony continues into the second scene, when Juliet tells her parents she is going to marry Paris. The audience knows that she has plans to sneak away and marry Romeo instead.
Scene IV has the most intense irony of the act, when the Nurse and Lady Capulet find Juliet unconscious in her bed and believe she is dead. The audience knows that she has drunk a potion to appear dead so that she may run away to be married to Romeo. The irony is further compounded by the friar's remarks that Juliet has gone to a better place. The audience is aware that the friar supplied the potion and has orchestrated the plans for Juliet to run away with Romeo. Thus, the better place of which he speaks is Mantua, not heaven.