In Act II, Scene 4, Mercutio is telling Benvolio about Tybalt's abilities in fencing. He also reveals in his description of Tybalt, that he fights with great precision. Mercutio compares Tybalt to a cat. Tybalt embodies the personality traits of a cat: inquisitive, confrontational, precise and skillful. It is ironic that Mercutio is describing Tybalt's ability to use a sword. Later in the play, in Act III, Scene 1, Mercutio and Tybalt fight each other in a duel, and in an attempt to stop the fight, Romeo jumps in front of them, and Mercutio is fatally stabbed by Tybalt.
Irony, simply put, is the opposite of what one expects. Technically, this type of irony is called situational irony. In this sense, then, it is ironic that Romeo informs Friar Lawrence in Scene ll:
With Rosaline, my ghostly father? No.
I have forgot that name and that name’s woe.
you would never expect romeo to stop being in love with rosoline