"The Sniper" is a fictional story based on the Irish Civil War in the 1920's.
Fiction is writing that is made up by the writer. The characters are not real people and the events are not presented as something that actually happened as the writer described.
While some stories are obviously fiction, like science fiction accounts set in the future or stories of knights and cavemen from the distant past, stories based on true historical events, such as wars, sometimes sound like real (nonfiction) pieces.
This is because writers of stories that are set in real historical events usually strive to make their stories seem very realistic. They want the reader to feel like they are really there, witnessing the event themselves. Sometimes they even make them sound like news reports. So stories like this “feel real” to the reader.
The line between fiction and nonfiction gets a little blurry sometimes when stories are based on true events. If the writer is creating a factually accurate account, we can call it nonfiction. If the writer is simply basing his story on an actual event but making changes, we call it fiction. If it is closely related to a real event, we may say that the story is a “fictionalized” account of a real event.