The rooftop in which the sniper is situation is one particular detail of the setting the contentious nature of time and place. The opening of the story illuminates this detail:
Around the beleaguered Four Courts the heavy guns roared. 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 Starters were waging civil war.
The Civil War in Ireland has caused Dublin to be "enveloped in darkness." This condition impacts the sniper on both a physical level and an emotional one, for he does not reflect as to who his target is. Rather, he focuses on them as "the enemy," something that will be repudiated by the end of the story.
Another detail that brings out the time and place of the setting would be the description of the sniper, himself. He is shown at the start of the story to be an object that almost transcends time and space, singularly focused on carrying out his mission no matter what:
On a rooftop near O'Connell Bridge, a Republican sniper lay watching. Beside him lay his rifle and over his shoulders was slung a pair of field glasses. 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.
The description of the time and place of the sniper brings out important details regarding his setting. In "thin and ascetic," one sees that the mission is his driving force, reflected in the "cold gleam of the fanatic." The "deep and thoughtful eyes" were ones that discarded contingency like time and place in favor of the mission that had to be embraced. In being accustomed to "looking at death," this detail brings out how time and space mean little to the sniper. The mission is all that counts, something that will undergo significant alteration as the narrative progresses.