What is the irony of Casca's description of the populace in Act I?
To answer this question, I'd like to expand on the other response and explore Casca's contempt for Rome's peasantry. In Act 1, Scene 2, Casca describes the common Romans' respect for Caesar, saying that "the rabblement hooted and clapped / their chopped hands and threw up their sweaty nightcaps / and uttered such a deal of stinking breath..." (248-50). Clearly, Casca has little respect for the masses of Rome, as he regards them as "rabblement" with "sweaty nightcaps" and "stinking breath." What's ironic about this reality is that, as a future member of the conspiracy, Casca should be acting in the interest of normal Romans, as the perceived point of the conspiracy is to protect Rome's political freedom from Caesar's tyranny. Clearly, however, Casca ironically has no love for the common Roman, and so probably cares little for the political freedom of the masses.
This irony reveals an important fact about most of the conspirators: their supposedly altruistic intention to protect the common Roman is a facade. Rather, most of the conspirators use this idea to justify their envy of Caesar's power and their desire to seize power for themselves. In reality, only Brutus truly acts out of the selfless desire to protect Rome's political freedom. The other conspirators are merely pretending.
The irony of Casca's description is that it shows he has a total contempt for the common people. He loses no opportunity to denigrate them as repulsive, mindless, and easily swayed:
If the tag-rag people did not clap him and hiss him according as he pleased and displeased them, as they use to do the players in the theatre, I am no true man.
He even remarks that they are dirty and have "stinking breath." The irony in this is that as a conspirator against Caesar, he will have to rely on the support of the common people to make sure that their cause prevails after Caesar is killed. Indeed, he is the first of the conspirators to urge Brutus to speak to the common people and explain their actions after the assassination is carried out successfully (Act III, Scene 1).