What is irony in romeo and juliet act II?

It's ironic that when Mercutio teases Romeo about being in love with Rosaline, Romeo is really in love with Juliet.

It's ironic that the Friar advises Romeo to "love moderately" as he is about to perform the very sudden marriage between the passionate young people.

Friar Laurence points out an irony as he gathers herbs: used correctly, poisonous herbs can cure disease, and vice-versa. This irony is mirrored in the play as whole, in which an good thing, the love of Romeo and Juliet, leads to their deaths, and a bad thing, the deaths of Romeo and Juliet, leads to a good thing, the end of the feud between the families.

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