Antony may be using sarcasm in his references to Brutus, but he really does regard Brutus as an honorable man. Evidence of Brutus's honorable character is obvious in the fact that he is permitting Antony to speak at Caesar's funeral. Antony is fully aware that it would be unwise to speak disparagingly of Brutus. One of the Plebians says: "'Twere best he speak no harm of Brutus here!" At the very end of the play, when Antony and Octavius are viewing Brutus's dead body, Antony says of his enemy:

This was the noblest Roman of them all.

It's also important to note that Anthony was still following the rules set forth by Brutus - one of those rules srating that he could not place blame on the conspirators for their actions. He uses the positive character word "honorable" in order to follow that rule, but through excessive repetition (borderline sarcasm), he is able to munipulate the crowd to "rage and mutiny".

Antony's speech is delivered to evoke pity from the Romans, but it is also meant to be a cynical response to Brutus' speech.
Throughout Brutus' speech, he repeats the word "honour" several times, in an attempt to prove to the Romans that he conspired against Caesar for the good of Rome ("...not that I loved Caesar less, but that I loved Rome more").
Antony is trying to turn Brutus' own words against him.