This is a scene of many contrasts (which is the broadest definition of irony): A wedding in one room and a death in another; a bridegroom who comes for a bride who has gone; a Friar who appears to grieve but who must secretly be thrilled that his concoction has worked. Dramatic irony is when we (readers or audience) know what the others do not. To that extent, these are all examples of dramatic irony.
According to The American Heritage Dictionary, dramatic irony is, the “effect achieved by leading an audience to understand an incongruity between a situation and the accompanying speeches, while the characters in the play remain unaware of the incongruity.” The chaos found in the Capulet household in Act IV scene iv is due to the fact that everyone is preparing for the wedding between Juliet and Paris. The dramatic irony is the fact that Juliet is upstairs dead and the wedding that everyone was so gleefully preparing for in the beginning of the scene becomes at funeral at the end. The audience is privy to Juliet’s suicide before the other characters in the play because it is necessary for the development of the theme. The audience needs to be aware of the reasons as to why Juliet killed herself so that they do not blow it off as a mindless suicide. That is why the dramatic irony is so important in this particular scene of the play.
While the reader or audience of Romeo and Juliet does know from the Prologue of Act I that the love of Romeo and Juliet is "death-marked," at this point in the play, however, the immediate dramatic irony--the only irony that exists within this particular scene--is the fact that Lord Capulet believes that "The bridegroom he is come already" (4.4.31) is Paris with whom he has arranged for Juliet to marry; however, "the bridegroom he is come already" assumes a different meaning for the reader or audience: Indeed, the bridegroom has already been there and consummated the marriage (in Act II), but it is Romeo Montague, not Paris.
Unwittingly, the character of Lord Capulet has said much more than that of which he is aware=dramatic irony.
Dramatic irony comes when we as the audience know more about what is going on (and what will go on) than the characters do. When this happens, we perceive things in a different way than they do.
In Act IV, Scene 4, the Capulet household is happy and excited. They all believe that there is going to be a wedding -- that Paris and Juliet will marry and, presumably, live happily ever after. So, to them, the events of this scene are exciting and optimistic and happy.
We, in contrast, know that Romeo and Juliet are going to die. So to us the bustle and excitement are just sad. We know that all of this excitement is just going to end in grief and disappointment.
Since we know more than they do, we understand the events of the scene differently than we do. Therefore, this is an example of dramatic irony.
Dramatic irony is a consequence of the readers or the audience possessing information which the characters on stage do not have.
In ActIV Sc.3 of "Romeo and Juliet" Juliet is left alone in her bedroom the previous night of her marriage to Paris. She soon overcomes her misgivings and drinks the potion which the Friar has given her. Its meant to put her to sleep for a very long time so that every one believes that she is dead. She is to wake up in the funeral chamber of the Capulets and be united with Romeo:
Dramatic irony results in the next scene because the entire household of the Capulets is busy making preparations for a wedding which is not to take place. The readers and the audience know that the wedding of Juliet and Paris will not be celebrated while everyone else in the Capulet household look forward expectantly to the wedding.