The irony in the last sentence of the story "The Sniper" is that the enemy from which the sniper had been protecting himself, that tries to kill the sniper, and that ends up getting killed by the sniper happens to be his own brother.
The entire story of "The Sniper" is about a soldier's fight to survive during the Irish Civil War. Already placed in a strategic position, the sniper's job is to observe any danger, and to shoot at sight, thus eliminating any potential enemy around the zone.
This particular sniper at first comes across as a cold-hearted killer. So cold-hearted in fact, that he even has the gall to eat a sandwich and drink his whiskey as he is monitoring the location.
However, during the fire exchange that ensues, we find a man with the same worries and fears of dying as any other man. Moreover, the sniper's image becomes more real to the reader when he cusses the war, and everybody in it out of frustration. To add to this, he becomes curious as to who he may have shot. He is sure that. whoever this man is, he must have known him at a happier time.
The fact that his victim is his brother shows us the cruelty, irony, and sadness of war: We are all brothers and sisters of the human race, and war makes us kill our own.
The sniper is fighting in the Irish Civil War of the 1920s which saw families split because they joined different sides - pro-treaty and anti-treaty. The story tells how the sniper is targeting an enemy soldier and then shoots him. The last line of the story is:
"Then the sniper turned over the dead body and looked into his brother's face."