Describe the setting of "The Sniper." Why do you think this setting is so limited?

"The Sniper" is set in the city of Dublin during the Irish Civil War. This places the story somewhere in 1922 or 1923.  Beyond this general setting, the bulk of the story takes place on a building rooftop near O’Connell Bridge where the protagonist has inserted himself.  We are told that night has just fallen.  That is basically all the information that readers are given for this story's setting.  As the question states, the setting details are fairly limited.  
There could be a couple of reasons for why the author chose to limit the setting details so much.  One reason could be basic genre conventions.  O'Flaherty is writing a short story, and a lengthy setting description works against his ability to keep the story intense and short.  
A more likely reason for keeping setting details to a minimum is that it fits better with what O'Flaherty does with other details in the story.  Very few details are given about anything in the story.  Other than a description of the protagonist's face and eyes, we do not get any other details about him.  We are not given an age, height, or description of clothing.  It is the same with the other characters as well.  Readers are given almost zero details about the old woman, the other sniper, or the man in the car.  In fact, O'Flaherty does not even bother naming the characters in the story.  Keeping the setting details to a minimum fits with the story's lack of detail regarding its characters.