Popilius is another Roman senator but is not a conspirator. However, from this quote which he says to Brutus and Cassius before the assassination, Shakespeare makes it obvious to the reader that more than just the conspirators know about the plot. It is likely that many of the other senators know of the plot for in real life there were many people in on the assassination than there was in the play.
Also, this quote we can infer that Popilius sides with Brutus and the rest. Therefore there is a divide in Tome: those for Caesar and those against.
When Popilius says this, Cassius gets very worked up thinking that Popilius is going to call them out, but of course he was overreacting.
This come from Act III. Brutus has just given his speech to the Roman citizens explaining why Caesar had to die. He leaves the pulpit and Antony is left along with the Roman citizens to make his funeral speech. He uses much verbal irony throughout his speech in order to warm up to the crown. In the quote above he is referring to the conspirators as "honorable men." Although his sarcasm is not evident at the beginning of his speech, it becomes much more evident as he continues to call them honorable even though he shows how great and wonderful Caesar was (although the "honorable" conspirators did not think so..)