Onomatopoeia is the formation of a word that imitates the sound to which it refers. Examples are cuckoo, meow, bang, and pow. In "The Sniper," O'Flaherty uses onomatopoeia in multiple places. He does this in order to give the reader a more detailed idea of the sights and sounds of a war zone. In the opening paragraph, the narrator notes that the guns "roared." The guns firing together and in the distance can have the sound of a roar. Just before engaging his enemy, the sniper takes a "whiff" from his cigarette. The sound of "whiff" is similar to the sound of inhaling a cigarette.
The second time he is shot at, a bullet "whizzed" over his head. A bullet can make a sound similar to how the word "whizzed" sounds. After shooting his enemy, the sniper drops his rifle and it is said to have "clattered on the pavement." The word "clatter" has a sound similar to the noise a metallic object might make when it hits pavement.
"The Sniper" is littered with examples of onomatopoeia (in which the writer uses a word that is phonetically similar to the sound it describes). In fact, O'Flaherty uses onomatopoeia in the very first paragraph when he describes the roar of "heavy guns." Similarly, in the fifth paragraph, he states that a bullet "whizzed" over the sniper's head. He also uses the word "whiff" to describe the sound of the sniper inhaling his cigarette.
O'Flaherty also uses onomatopoeia to enhance the description of the death of the second sniper. The rifle "clattered" on the pavement, for example, while the body hit the ground with a "thud."
By using this literary technique, O'Flaherty effectively recreates the sounds of the Irish Civil War. As a result, he is able to bring to life the very violent setting of his story and emphasize to his readers that this period was indeed one of the most turbulent in Irish history.