What details in the first paragraph help establish the atmosphere or mood of "The Sniper" by Liam O'Flaherty?

Liam O’Flaherty establishes the tense, ominous, violent mood of the story in the first paragraph by describing the atmosphere of Dublin under siege during the Irish Civil War. O'Flaherty depicts the twilight sky, which corresponds to the dark, dangerous conditions in the "beleaguered Four Courts" as the Republicans and Free Staters engage in battle. The dim atmosphere and continuous sound of artillery fire throughout the city also adds to the suspenseful, threatening atmosphere of Dublin.
As the night sky darkens and the waters of the Liffey become black, enemy soldiers shoot at one another while night gradually approaches. O'Flaherty's ominous mood is fitting for a short story focusing on the Irish Civil War. Throughout the short story, O'Flaherty explores the effects of the civil war on Irish citizens, soldiers, and family members. In the first paragraph, O'Flaherty establishes a suspenseful, ominous mood, which correlates to the dark, gloomy atmosphere of the beleaguered city during the Irish Civil War.

Let's start with the mood and atmosphere of the story as a whole.  I would say that this story's mood is tense, dark, and violent.  In a nutshell, the story is about a sniper that coldly shoots and kills three people.  One of them happens to be his brother.  
The opening paragraph helps to establish that mood by setting the story in the evening.  Day is fading into night, so there is an actual darkness that accompanies the emotional darkness of the story.  

The long June twilight faded into night. Dublin lay enveloped in darkness but for the dim light of the moon that shone through fleecy clouds, casting a pale light as of approaching dawn over the streets and the dark waters of the Liffey.

Notice all of the words that focus reader attention on darkness and dark times.  "Twilight," "faded," "night," "darkness," "dim," and "pale" all highlight an ominous and oppressive feeling.  The rest of the opening paragraph points readers toward the violence that is happening around the setting.  A civil war is going on and guns can be constantly heard.  Based on the opening paragraph, readers know that this story is going to be a dark and violent story.