Why was lighting a cigarette risky for the sniper?

In Liam O'Flaherty's short story, "The Sniper" (1923), the reasons stated by the first-person narrator, the sniper, are shown specifically in this passage from the story:

Then he paused for a moment, considering whether he should risk a smoke. It was dangerous. The flash might be seen in the darkness, and there were enemies watching. He decided to take the risk. Placing a cigarette between his lips, he struck a match, inhaled the smoke hurriedly...

In Liam O'Flaherty's short story, "The Sniper" (1923), the reasons stated by the first-person narrator, the sniper, are shown specifically in this passage from the story:

Then he paused for a moment, considering whether he should risk a smoke. It was dangerous. The flash might be seen in the darkness, and there were enemies watching. He decided to take the risk. Placing a cigarette between his lips, he struck a match, inhaled the smoke hurriedly and put out the light. Almost immediately, a bullet flattened itself against the parapet of the roof. The sniper took another whiff and put out the cigarette.

Instantly, the story shows that the sniper's concerns were all legitimate and that lighting the cigarette has endangered his life. 

In general terms, lighting a cigarette would be risky for a sniper because by definition a sniper is someone who is trying to remain hidden and unseen while shooting at another human.

If the sniper is operating during the daytime, smoke might be seen by the person being targeted and give away the sniper's position. If at night, both the flame created by a match or lighter, and the glow at the end of the cigarette could be seen.

With today's technology (not available to the sniper in O'Flaherty's story), if human targets are aware of potential snipers in an area, they could also have heat-tracking equipment which would pick up the heat from a person in hiding or from a match, lighter, or cigarette. In addition, the sniper could be distracted by the light of their own match and not see their target if the person happened to come into view. These are general reasons it is risky to light a cigarette if a sniper wants to remain hidden.

In O'Flaherty's story, the sniper is a member of the Republicans, aiming at an unknown Free-Stater during the Irish Civil War. The action occurs in the city of Dublin. When the sniper successfully kills his enemy, he discovers the dead man is his own brother. This story was published in a London magazine called The New Leader.