how does mercutio describe benvolio? why is his description ironic?

Are you referring to act 3 sc 1? Mercutio says that Benvolio is moody. He claims that although Benvolio may look like a peace-maker, he actually can get angry and, deep down, is looking for a good fight. I don't think Benvolio is a fighter. He tries to make peace throughout the play. His sensitivities are much more peaceful ahan violent.