The antagonist of the short story is the opposing Free Stater sniper stationed on the opposite rooftop. The Free Stater sniper is considered the antagonist because his actions create the primary man versus man conflict in the story, which the Republican sniper must overcome. Throughout the story, the Free Stater sniper attempts to kill the Republican sniper, who is forced to create an elaborate ruse to defeat him. The protagonist of the story successfully tricks the Free Stater sniper into believing that he is dead, which gives him the opportunity to take a kill shot. After killing the enemy Free Stater sniper, the protagonist discovers that the antagonist was his brother when he looks into his lifeless eyes on the street.
In “The Sniper,” it's the rebel Republican sniper, a member of the anti-government forces, who's the protagonist. This simply means that he's the main character in the story, and his actions are the most significant. His antagonist is the enemy sniper, stationed on the opposite roof. He's fighting on behalf of the Free State forces, which are those troops that are loyal to the government in the Irish Civil War. They see their role as defending the new Irish state from rebels and are hostile to what they see as a sell-out to the British.
Unbeknownst to the protagonist, the Free State sniper he ends up killing is actually his brother. This was quite a common occurrence during the Civil War, when friends were pitted against friends, brothers against brothers, in this particularly bitter and bloody conflict. The shooting of the protagonist by his brother aptly symbolizes the fratricidal nature of civil war.
As we saw earlier, one of the factors distinguishing a protagonist from an antagonist is that the former's actions tend to be more significant. And that's certainly the case here. For it's the killing by the protagonist of his antagonist, his brother, that is by far the most significant action in the story as it serves to highlight the terrible nature of civil war and how it splits friends and families apart.
The protagonist is the main character and the focal point in a piece of literature. In contrast, the antagonist is the character set up in conflict with the main character. After the Anglo-Irish Treaty of 1922 which made Ireland a part of the British Commonwealth, the Irish Republican Army split into two factions. The Republicans vehemently opposed the treaty while the Free-Staters were in agreement with the British that Ireland should fall under British hegemony.
Liam O'Flaherty's short story "The Sniper" is a snapshot of the war which ensued between the Republican and Free-State troops. O'Flaherty makes a Republican sniper the protagonist and focus of his story. The protagonist, who goes unnamed, is described as "thin and ascetic" with "the cold gleam of the fanatic." He is positioned on a Dublin rooftop during the Battle of Dublin in June and July of 1922. He inevitably comes into conflict with a Free-State sniper who is positioned on a nearby rooftop. This Free-State sniper is considered the antagonist in the story. Ultimately, the protagonist kills the antagonist but later discovers, in an extreme example of irony, that the enemy sniper was really his brother.
In the story The Sniper by Liam O'Flaherty, the characters are in the middle of the Irish Civil War where families divided over the issues much like they did in the American Civil War. The protagonist is the main character, the one who learns something, the one who carries the weight of the story. In this case it is the Republican sniper waiting on the rooftop who is the protagonist because he is the main character which the story is about who learns the sorrow of war, and who carries the story to the end. The antagonist is usually thought of as the opposing person or force which fights against the main character or the protagonist. In this story the antagonist is the sniper who fights against the main character by trying to kill him. Instead he gives away his position by the flash of his rifle, is shot and killed by the other man who in a moment of curiosity, gets off the roof, turns over the dead man, and discovers that he has killed his own brother.