Why is it ironic for Cassius to die in battle?act 5 scene 1 in julius caesar
First, in Shakespeare's Julius Caesar, Cassius doesn't really die in battle. He has his servant run him through with his own sword when he thinks the battle is lost. Also, is some ways, it isn't ironic. He is on the losing side of a conspiracy. His fate would be either death or capture, in which case he would most likely be paraded around Rome as a prisoner.
That said, in some respects one could argue his losing his life as a result of losing the battle is ironic for a few reasons.
First, though he does not possess the nobility of Brutus, he certainly makes better decisions. He tries to convince Brutus to kill Antony at the same time Caesar is assassinated. He also tells Brutus not to let Antony speak at Caesar's funeral. Finally, he tells Brutus to keep the armies in the easily-defended defensive position they are in before the final battle, but instead Brutus orders the armies to move and attack.
Had Brutus listened, or had Cassius been in charge, the conspiracy would, or at least might, have been successful.
It is ironic, then, that Brutus dies after losing the battle, when the battle might never have needed to be fought.