The short story A Horseman in the Sky takes place in the fall of 1861 during the Civil War in Western Virginia. A Union soldier has fallen asleep at his post of duty. He is near the edge of a high cliff that overlooks a deep valley (I couldn't find the greatest scene that showed the setting).
Was there anybody on the horse?
The main character is Carter Druse who is a member of the Union army. We know that he is the son of wealthy parents, is an only child, and that he grew up only a few miles from where the story takes place. At this point in the story, he has realized that he had fallen asleep at his post of duty which is penalized by death.
Carter sees a Confederate soldier at the top of the cliff that has seen the waiting union ambush. He is "an enemy more formidable for his knowledge than all his army for its numbers" as the text states. Carter has an internal struggle (character vs. self) and contemplates not killing the man, but gives in to the call of duty shooting the man's horse, sending them both off the side of the cliff.
After about ten minutes, Carter's sergeant creeps up to where he is keeping watch. He asks if he fired and Carter tells him what happened. Then, the sergeant and the audience learn who was on the horse that went off the side of the cliff.
I do not think there was any true irony in the story. Someone who doesn't really know what irony is might say that it is ironic that the man on the horse turned out to be Carter's father. However, there was no sarcasm (verbal irony) present, we knew as much as the characters did (no dramatic irony), and there wasn’t any situation or event that was contrary to what was expected (situational irony).
There are many possible themes for a story like this. I think one theme for A Horseman in the Sky is that war is a terrible thing that can divide families. Another theme could be that if your duty is to a bigger cause, it is not always easy to do what is expected of you.