How is "Mending Wall" ironic?

Robert Frost's poem "Mending Wall" is told in the first person. The narrator describes the task of maintaining a wall between the neighbor's pine trees and his own apple orchard. The wall is difficult to maintain. It is a dry stone wall that partially collapses due to snow and freezing in winter and parts sometimes get knocked down by hunters. The narrator speculates that there is no real reason for the wall's existence, as there is nothing to be walled in or out, and the wall is neither high nor durable. 

The main irony in the poem has to do with the phrase the narrator's neighbor repeats, "Good fences make good neighbours." On the one hand, it seems odd, as fences separate people. The narrator speculates, though, that in the case of dairy farmers, a wall prevents mingling of animal herds and ensuing disputes. The irony is that although the narrator and his neighbor have little in common, the shared annual duty of mending the wall brings them together, and thus maintaining good fences, does, in fact, serve to make them good neighbors by letting them bond over this shared task.