Which literary devices does O'Flaherty use to communicate his theme in "The Sniper"? (Please provide examples from quotes.)

First, there are some important main themes in this story: the visceral nature of physical conflict, the finality of death, and the dehumanizing nature of war.
In the story, the author uses anaphora to emphasize the visceral nature of physical conflict and the dehumanizing nature of war. Anaphora is the repetition of a word or phrase at the beginning of successive sentences.

He was eating a sandwich hungrily. He had eaten nothing since morning. He had been too excited to eat.
The sniper raised his rifle and fired. The head fell heavily on the turret wall. The woman darted toward the side street. The sniper fired again. The woman whirled round and fell with a shriek into the gutter.
The woman’s corpse lay still in the gutter. The sniper lay still for a long time nursing his wounded arm and planning escape.
His teeth chattered, he began to gibber to himself, cursing the war, cursing himself, cursing everybody.

In the above quotes, the author emphasizes that war has very real consequences for everyone. Soldiers fight to the death, and everyone endures privation, fear, and extreme provocation at every turn of the conflict. Strong emotions take hold when death threatens. Grim actions occur in split seconds, emphasizing the sometimes furious pace of war.
The author also uses similes and metaphors to characterize the dehumanizing nature of war.

Here and there through the city, machine guns and rifles broke the silence of the night, spasmodically, like dogs barking on lone farms. Republicans and Free Staters were waging civil war.
His bullets would never pierce the steel that covered the gray monster.

In the first quote, the author uses a simile to equate the noise of machine guns and rifles to that of "dogs barking on lone farms." The simile emphasizes the stark reality of the deafening sounds of battle. In the second quote, the author uses a metaphor to describe the enemy's armored vehicle. The tank is like a gray monster, capable of inflicting maximum damage on those who dare to challenge it.
The author also uses visual imagery to highlight all his main themes.

The blood was oozing through the sleeve of his coat. There was no pain—just a deadened sensation, as if the arm had been cut off.
The armoured car had retired speedily over the bridge, with the machine gunner’s head hanging lifeless over the turret.
His face was the face of a student, thin and ascetic, but his eyes had the cold gleam of the fanatic. They were deep and thoughtful, the eyes of a man who is used to looking at death.

In the first quote, we see the blood oozing through the sleeve. The wound is painful and debilitating. The sniper finds himself weakening. In the second quote, we see the gunner's head hanging over the turret. These are disconcerting images. Essentially, war continues, without respect to the dead or injured. In the third quote, we get a "glimpse" of the sniper. He is thin, and his eyes have the "cold gleam of a fanatic." The dehumanizing aspect of war is emphasized in the imagery of a soldier who must accustom himself to "looking at death."
https://rhetorica.net/tropes.htm