As for tone and mood, consider that the story is centered around a lone sniper, a very young soldier trying to survive in the Irish Civil War. Now look at the opening paragraph and note the repetition of the word "dark" and "night" as well as words such as "faded," "dim," "silent," and guns "barking." The scene is dark, creepy, and silent; that silence is broken only by gunfire. The mood is dark, foreboding, and always filled with tension in the anticipation of violence.
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 Liffey. 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.
As the story progresses, the mood and tone follow the sniper's emotions and reactions to his situation. Initially, he is described as having the appearance of a young man (a student) but with the eyes of a fanatic. He is young enough to be innocent but has been conditioned to be a soldier. Just like the description of the opening scene, the sniper experiences loneliness, tension, and violence. He is initially desensitized to the violence required of war, re-sensitized, desensitized again, and finally he faces his ultimate fear: staring into the face of his victim/brother. The mood fluctuates between these feelings: darkness, tension, and violence (and lastly, reflection).