What is irony in the poem "Mending Wall"?

In Robert Frost's poem, "Mending Wall," the speaker considers the idea of the wall in his yard and wonders why it is necessary. He prefers that there is no wall between he and his neighbor so they can have access to each other. However, the neighbor feels otherwise and believes that "good fences make good neighbors." He believes that walls or fences set up healthy boundaries between neighbors and that keeping an element of privacy makes for better relationships.

The irony is that putting a wall up between yourself and someone else seems like it would do the opposite--it seems like it would create a barrier, distance. For some people, it would. The speaker, for instance, thinks no fences would make good neighbors. Yet, the neighbor prefers a barrier, regardless of how ironic it may seem. For him, that is what makes him comfortable and makes him feel more neighborly towards the speaker.