How is "Mending Wall" ironic?

Walls are generally built to protect and secure. They are there to either keep out what is bad or unwanted, or to keep in what is valuable and important. Ironically, though, it seems that the wall between the speaker and his neighbor serves no purpose. The speaker states in lines 22 to 23:

It comes to little more:There where it is we do not need the wall....

The speaker is clearly aware of the fact that they do not need the wall. He emphasizes this:


He is all pine and I am apple orchard.My apple trees will never get acrossAnd eat the cones under his pines....

The speaker considers it absurd that his apple trees would ever cross over into his neighbor's property and feed on "the cones under his pines." He also muses that


Before I built a wall I'd ask to knowWhat I was walling in or walling out,And to whom I was like to give offence.

It seems however, that his neighbor does not share his sentiment and stubbornly refuses to acknowledge the speaker's entirely logical argument. Even though the speaker has obviously stated that the wall would only have served a purpose if they had cows, his neighbor is determined to maintain the barrier. This creates further irony since the two neighbors, it seems, regularly go about fixing whatever damage—from natural or other causes—the wall has suffered when there is actually no need to do so. 
 
This act of neighborliness introduces more irony because the one thing that literally keeps them apart is also that which brings them together. They are involved in a joint act when fixing the damage.
 
The speaker seems somewhat resentful about his neighbor's obviously obstinate stance:


He will not go behind his father's saying,And he likes having thought of it so wellHe says again, "Good fences make good neighbours."

It appears as if the neighbor is a staunch believer in his father's principle of "Good fences make good neighbours." The speaker adopts a cynical tone in this regard and sees the neighbor's insistence as something uncivilized, somewhat aggressive and sinister. He mentions:


I see him thereBringing a stone grasped firmly by the topIn each hand, like an old-stone savage armed.He moves in darkness as it seems to me,
Not of woods only and the shade of trees.

The speaker seems to believe that the neighbor has a less friendly ulterior motive for retaining and maintaining the wall. This is, in itself, also ironic, for in the speaker's eyes, there is nothing good in maintaining a useless partition when, if they are such good neighbors, they don't need to be separated at all.  

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.