After Cassius with Brutus and Antony with Octavius parley on the battlefield, Cassius takes his servant Messala aside and reveals to him the ironic truth that that day was also Cassius's birthday, as we see him state to Messala in the lines, "Messala, / This is my birth-day; as this very day / Was Cassius born" (V.i.72-74). The term irony can be defined as any "outcome of events" that is the exact opposite of what was expected (Random House Dictionary). Normally on one's birthday, one expects to celebrate one's birth; however, Cassius rightly predicts that that day will also be the day he dies. Hence, ironically, he is not just thinking about the day of his birth, he is thinking about the day as the day he dies.
We can tell that Cassius rightly believes he will die that day because he next speaks of omens, saying that he once agreed with the Greek Philosopher Epicurus who disbelieved in omens but Cassius now strongly believes in omens instead. He next goes on to describe a vision he saw that he interprets as an omen, a vision the day before of two eagles who perched on the army's flag and ate out of the soldiers' hands as they marched on to Philippi but vanished in the morning, replaced by "ravens, crows, and kites," all known to be birds that feed on the flesh of the dead. As a result of this vision, he believes the carrion birds are waiting for the whole army to die and rightly understands the vision to be a prophecy of his and the whole army's deaths, as he states in the lines, "Their shadows seem / A canopy most fatal, under which / Our army lies, ready to give up the ghost" (88-90). Hence, he also sees just how ironic it is to expect himself to die on the exact same day that he was born.
The irony that Cassius presents Brutus with after the fight that hat is it his birthday. whispers when something opposite happens from what is supposed to happen. you would expect someone's birth would occur on a birthday but Cassius predicts this is also the day that he would die. It's ironic that by the exact day that he was born he would also die