Foreshadowing, or the deliberate planting of hints as to what will come later on in the novel, is a deliberate strategy that is used very successfully by Liam O'Flaherty in this excellent short story. Continually foreshadowing is used to point towards what happens later on in the story. One of the first examples comes when the protagonist decides whether to risk smoking or not:
He paused for a moment, considering whether he should risk a smoke. It was dangerous. The flash might be seen in the darkness, and there were enemies watching.
This of course foreshadows the first bullet that is fired at him when he lights the cigarette, which reinforces the position of danger that he is in.
We could also say that the initial description we are given of the sniper foreshadows the killings that he commits. Consider that we are told his eyes "had the cold gleam of the fanatic." In addition, we are told that his eyes are the "eyes of a man who is used to looking at death." This of course foreshadows the ease and coldness with which he kills both an informer, a soldier, and his anonymous opponent on the roof opposite him.
Lastly, the most important example of foreshadowing in the story comes when he has triumphed over his mystery opponent on the opposite roof and instead of feeling happy, is overwhelmed by disgust and fear:
His teeth chattered, he began to gibber to himself, cursing the war, cursing himself, cursing everybody.
This of course foreshadows the most traumatic revelation of the story; that the identity of his opponent is actually his brother.