Irony is a literary term for the difference between what is real and what is just appearance, as well as the difference between what the reader expects and what he actually gets.  William Shakespeare, the author of Romeo and Juliet, often incorporated irony in his works.  Dramatic irony, which is found in several place in Romeo and Juliet, occurs when there is a difference between what a character believes and what the audience knows to be true.

In Act IV of Romeo and Juliet, Juliet Capulet flees to Friar Lawrence's cell in search of a plan to overcome the obstacles facing her relationship with Romeo Montague; Romeo killed Tybalt, Juliet's cousin, only a few hours after he and Juliet were married in secret (by Friar Lawrence).  When Juliet reaches Friar Lawrence's cell, she is surprised to find County Paris there. 

Paris has come to Friar Lawrence in order to ask the Friar to perform a the marriage ceremony for himself and Juliet.  Paris, who is infatuated with Juliet, does not realize that Juliet is already married and believes that her sadness is due only to the death of Tybalt.  He attempts to share his feelings of love with Juliet who, for obvious reasons, has no interest in him.