Clearly suspense is a very important element in this short story. From the very first paragraph the suspense that keeps us reading builds until we reach the shocking ending of this tale. Note how this is achieved:
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 Staters were waging civil war.
This description does much to paint the scene for us: as we read on we are plunged in to a civil war where brother fights brother across the rooftops of the city. Then we are introduced to the main character - a Republican sniper who is lying on a rooftop. Note what is said to us:
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. He decided to take the risk.
Note how the author builds up the suspense here by telling us about the potential dangers of the simple action of lighting a cigarette before the sniper does so. This thus sets the reader on the edge of his or her seat as we wait to see what happens when he lights his cigarette. The gun shot that occurs as a result thus shocks us but also raises suspense in turn as we discover that there is a sniper on the opposite roof trying to kill the main character. This thus makes us wonder who is going to survive this urban battle of wits and come off the roof.
Suspense then can be said to be sustained through the setting and the description, but most importantly the way that the author describes the potential dangers of the situation the character finds himself in.