There are two elements of irony in this important part of the play when Romeo and Juliet first meet each other in Act I scene 5. The first is the way in which Romeo is completely overwhelmed by instantaneous love for Juliet, in spite of all of his love-sickness for Rosalind that he protested earlier on in the play. Note what he says:
Did my heart love till now? Forswear it, sight!
For I ne'er saw true beauty till this night.
In spite of how his love for Rosalind sends him into a complete mood and makes him withdrawn and depressed, Romeo suddenly forgets about Rosalind and becomes obsessed with Juliet.
The second element of irony is clearly dramatic. We know the identity of the object of Romeo's affection, even though he does not. We know that the Capulets and Montagues are engaged in a feud that makes such a love impossible. And yet, when Romeo inquires after Juliet's identity, he is not given a definite answer, and thus he thinks he is free to continue loving Juliet without knowing of her true identity.