Why is it ironic that Antony states in his funeral speech that he is "no great orator as Brutus is"?

That would be because Antony is clearly the better speaker. Brutus is clearly outmatched at the funeral speech. Antony is a master of rhetoric; he seldom means what he says. Antony's famous “Friends, Romans, Countrymen Lend Me Your Ears” speech is a masterful use of rhetoric to turn the crowd against the conspirators. Poor Brutus never had a chance.