Dramatic irony is a contrast between the character's own limited understanding of circumstances and the audience's greater understanding of the character's situation. As the events unfold, the character's limited understanding creates a moment of drama that the audience has already ironically anticipated.
Instances of dramatic irony are certainly more subtle in Act 2 than in other acts, but one example of it can be seen in Friar Laurence's lack of full understanding of the situation before he marries the couple. One thing Friar Laurence fails to learn is that Juliet's parents already plan to marry her to Count Paris; Juliet's mother is especially keen on the idea. Not only that, Juliet has already played the role of the dutiful daughter and promised to consider Paris. When Friar Laurence promises to marry Romeo to Juliet that day as he feels the marriage may unite the two feuding families and "may so happy prove / To turn [their] households' rancour to pure love," he also feels a sense of foreboding (I.iii.94-95). He feels that the marriage may prove to be a mistake and that some misfortune may happen as a result, as we see in his lines, "So smile the heavens upon this holy act / That after-hours with sorrow chide us not" (II.vi.1-2). While Friar Laurence may have a sense of foreboding, what he doesn't know is that Paris's desire to marry Juliet, as well as her parents' desire that she marry Paris, should have been a large factor in his decision to marry the couple and will play a huge role in the upcoming tragedies. However, the audience is well aware that Paris' and Juliet's parents' interests will prove to be a problem, and had Friar Laurence been aware that Juliet may become engaged to Paris, he might have acted differently.
A second example of dramatic irony can also be seen in relation to Friar Laurence as well. From the very First Prologue, the audience is well aware that the story will not have a happy ending for the couple. The audience is aware that two "star-cross'd lovers" have been born to two feuding families that the "pair of star-cross'd lovers take their life" (First Prologue, 6). Hence, when Friar Laurence becomes excited about the prospect of conducting the marriage because he sees it as a way to create peace in Verona, the audience is well aware that he is being overly confident. Likewise, when Friar Laurence begins to express doubt and hope that heaven will not chastise the marriage later with "sorrow," the audience feels the sense of pending doom that Friar Laurence is only vaguely aware of.
Dramatic irony is when the audience or reader knows something that a character does not. For example, in Act 2 we know that Romeo and Juliet have been married but their families do not.