The irony in "Mending Wall" comes from the two possible reasons for which the mending is undertaken. Depending on which reason for the mending you apply to the poem, you arrive at two completely different stories.
"Mending" sometimes refers to fixing something, as in sewing a torn piece of cloth to bring it back together. When friends mend an argument, they stop disagreeing and resume a cordial relationship. When Frost writes,
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.
he gives the impression that he and his neighbor are engaged in a friendly annual ritual with no more significance than that some of the wall's rocks have been dislodged by the winter's heaving of the ground or by hunters.
It would also be possible to read the "mending" as restoring the separation between the two neighbors. Although they have different types of crops on their respective sides of the fence - apple trees on one, pine trees on the other - and there would be no harm to either crop if the fence was eliminated, the neighbor is quite determined that the fence needs to be preserved in place and intact.
He will not go behind his father's saying,
And he likes having thought of it so well
He says again, "Good fences make good neighbors."
Is the mending process rebuilding the fence or reestablishing the division between the neighbors? Therein lies the irony.