"...The gaps I mean,
No one has seen them made or heard them made,
But at spring mending-time we find them there.
I let my neighbor know beyond the hill;
And on a day we meet to walk the line
And set the wall between us once again." (Ln. 9-14)

It is ironic that even though the author wishes nothing more than for the wall to come down, every year he continues to rebuild and reinforce the wall.

Irony