In "The Moose and the Sparrow," the characters are named in accordance with their perceived levels of power. When the story ends with an unexpected twist that upsets the established power dynamic, the reader's perception of that power is inverted and the names become ironic.
Moose, an archetypal "brute," is a big, strong lumberjack. He's aggressive and unintelligent, often brutalizing Cecil to punish him for his perceived weakness. Since moose are large, strong animals with the potential to cause serious harm, the name "Moose" is a reference to this imposing, threatening role.
In Cecil's case, the "Sparrow" epithet suggests the opposite. Cecil is characterized as small, delicate, flighty, and non-threatening, just like a tiny little bird. Because birds are agile and clever, this also could be interpreted as gently foreshadowing the story's end.
At the story's climax, Cecil leverages his intelligence to send Moose to his death. This paradigm is then shifted, and the irony is revealed—the Sparrow has used his intelligence to physically best the Moose, despite their dramatic disparity in physical strength and power.