In Scene 2 of Act I of Romeo and Juliet, a servant of Lord Capulet goes up to Romeo and Benvolio for help with reading an invitation list. The irony is when the servant says SERVANT. My master is the great/ rich Capulet; and if you be not of the house of Montagues, I/ pray come and crush a cup of wine (1.2.79-81). The audience knows Romeo and Benvolio are of the Montague house but the servant doesn't.
In Scene One of Act II in Romeo and Juliet, Mercutio jokes about Romeo's love sickness. Mercutio and Benvolio believe that Romeo is in love with a girl named Rosaline when the irony is that he's in love with Juliet instead.
In Scene 5 Act IV, Lady Capulet believes Juliet is upset because of Tybalt's death when Juliet is really crying over Romeo's exile.
In Scene 2 Act III, the irony here is when Juliet mistakes the Nurse for saying Romeo was dead when in actuality the Nurse is talking about Tybalt's death.
In Scene 1 Act V, the irony is Balthasar telling Romeo that Juliet is dead when Juliet is actually alive and faking her death.
In Scene 2 Act V, the irony is that Friar Laurence believes Romeo i still in Mantua when Romeo is actually coming back to Verona to be with Juliet.