A female sniper is typically called a "sniper" just like a male sniper. There isn't a separate term specifically for a female sniper. Both men and women who are trained in long-range precision shooting are referred to as snipers. Amateur but enthusiastic student of history.

The sniper is on the Republican side of the Irish Civil War. O'Flaherty's description of him suggests that he has been fighting for a long time the sniper has deep and thoughtful  eyes that were used to looking at death  (Paragraph 2). His actions are deliberate and quick.