What is ironic about the speaker in mending wall to maintain the wall?

What is ironic about the speaker in "Mending Wall," by Robert Frost is that he helps maintain the wall but he sees no point in having a wall. One grows pine trees and the other apple trees, so there is no need to separate because, as the speaker says, "My apples will never get across and eat the cones under his pines."

His neighbor says, "Good fences, make good neighbors," but for the speaker, if you are good neighbors, why would you need a wall? It only separates the good neighbors, if they were indeed good neighbors. 

The answer here would be "work ethic."