why is the irony concerning all of the fighting that goes on at the western front ?

This is a pretty big theme in the novel. Paul often wonders why the leaders, who inflict the wars on others, don’t just fight it out themselves. Men are dying by the thousands; virgins with rifles going to their graves. At the same time generals and political leaders, responsible for this, are sitting in safety somewhere else. Paul also feels that it is ironic that he has more in common with his enemy on the field than he does with civilians at home. Their shared experience of horror makes them (British/French) more brothers than villains.