There are several good summaries and details of the plot on the Enotes guide linked below.
The story begins by introducing the city of Dublin, the capital of Ireland, embroiled in a civil war. Night has fallen, but the fighting is relentless, and scattered all over the city. One participant is a sniper, who is young, but apparently this isn't his first battle.
The sniper hasn't eaten all day, and is taking a break, and decides to risk lighting a match to smoke a cigarette. Unfortunately the light from the match is seen by an enemy gunman, who pins down the sniper.
The sniper observes an armored car on the street below, and the men inside begin talking to an old woman, who is clearly telling them that the sniper is on the rooftop. The sniper shoots the woman and one of the men, but in exposing himself he is also shot by the other gunman.
The sniper binds his wounds but realizes there is now no way for him to get off the rooftop without being shot again, and because his arm is injured he can't use his rifle. Instead, he puts his hat on the end of his rifle and uses it as a decoy, then pretends to die when the gunman shoots his hat. However, the sniper still has a pistol, and he shoots the other gunman when his guard is lowered.
Curiosity inspires the sniper to go see who it was that he shot; it turns out to be his own brother.