The most memorable aspect of the story is the final revelation that the enemy sniper was the Republican sniper's brother, which changes some of the context and gives the story a deeper meaning that is left intentionally ambiguous.
The story also begins with some pretty blatant exposition, stating that the city is in a state of civil war, fought between the Republicans and Free Staters.
There are a few other insights but I think they're largely circumstantial, and it's a little hard to project their meaning with full confidence, or determine if they are truly representative of the factual war.
- I would assume that both sides are in either a defensive position or a war of attrition in this part of Dublin, since it would make little sense to deploy a single sniper with no support in an offensive campaign.
- The sniper does not hesitate to kill the old woman, supporting the idea that he is highly partisan and this is not the first time he has had to kill a civilian (if he even considers anyone a civilian in this war).
- The narrator mentions a "split in the army" suggesting that many combatants probably had formal military training and equipment, as well as personal familiarity with their rivals. This might also explain the sense of stalemate, since both sides would be equally matched.
The fact that each soldier fights for what he believes is right is the (seeming) basis of the plot. However, civil war pitted neighbor against neighbor and, in this case, brother against brother. It is only after killing his own brother that the soldier realizes the true impact of war—death. In an instant, the principles once held important enough to go to war for become insignificant when the price of "winning" is the death of another—especially the death of a loved one. The real price of war is horrific.