"The Sniper" is about the Irish Civil War between the Republicans and Free Staters, and we learn in the story that the war has separated friends and family. At the end of the story, the sniper discovers that he has killed his brother. With all this in mind, I would assume that in a sequel, O'Flaherty would pick up on plot development in the first story, and capitalize also on the clues left about the sniper's mindset at the end of the story.
Although the sniper is a fanatic—someone who is radically dedicated to a cause—when he first shoots the other sniper (and still does not know the other sniper's identity), his first reaction is anger—anger at the war and all the senseless death that comes with it. (This is when he throws his revolver on the ground, it misfires, and it almost takes his head off.) Before he even knows whom he has killed, his reaction indicates that the constant fighting, while a challenge, is taking its toll on the man's peace of mind. Perhaps, too, he is arriving at a place, especially after his brother's death at his own hand, that he will start to look within to take stock of where his life is going, and whether or not he will have any kind of future that does not deal with the killing of others or, ultimately, his own death.
In a sequel, I would anticipate the sniper's devastation at what he has done. Not only will he have to face himself in the mirror, but also his family—his parents—and his friends. What could he possibly say that could in any way justify his brother's death, in his own mind?
It would make sense, then, that the sniper would separate himself from the military. Because the war rages all around him and it is nearly impossible for civilians, as well as men try to forget the past, to separate themselves from the bombings and shootings that occur every day, I would expect the sniper to pack his things and move to the United States. (He could go anywhere, but I would think work might be easier to find in the US at that time—just prior to the Depression in the late 1920s.)
In the US, or any other country, the sniper would be able to begin again. He could probably get work as the Industrial Revolution has arrived in the US. He would be able to find a place to live, work hard, maybe meet someone, get married, and build a life that has the promise of a future. However, in order to fight the ghosts that would haunt him, I would expect that he would have to find something bigger than himself to help make the world a better place. It might come in the form of helping others, or finding comfort in religion. Only in this way do I feel he could find some sense of inner-peace.
If I were to write a sequel to "The Sniper", or guess what one might be about, I believe the sniper would try to put his past behind him and start over somewhere else.