The main character, "the sniper," is a Republican. The Irish Civil War was fought between Republicans and Free Staters from 1922-1923.
Prior to the Irish Civil War, the Irish War of Independence was fought between the Irish Republican Army and British forces from 1919-1921. The Irish Republican Army was fighting for Ireland to be a free, sovereign nation. They eventually came to a truce and the war resulted in the Anglo-English Treaty. The treaty gave Southern Ireland (5/6 of Ireland proper) some degree of independence. This was then called the "Free State." However, some members of the Irish Republican Army believed the treaty was a step backwards because it still included British intrusions and the future threat of British occupation of Ireland. This led to the Irish Civil War: Republicans versus the Free-Staters (who supported the treaty with the British).
The sniper fought on the side of the Irish Republicans. This is the group that opposed the Anglo-Irish Treaty because they thought it did not do enough to separate Ireland from the British.
Both groups were fighting for Irish independence. The pro-treaty Free-Staters simply thought that the treaty was the best they could get at that time. The Republicans thought otherwise. Irish Republicans occupied the Four Courts (the center of Dublin's court system) in the spring of 1922 and this was an early staging point of the Irish Civil War. This is noted in "The Sniper:"
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.