In mending wall the speaker does not see the necessity of the wall what does the speaker do that could be considered ironic

The action of the speaker that can be considered ironic is that actually the speaker takes initiative of establishing the wall up to himself but then he questions it and has some doubts about that. There is another irony,  I would say, the main one, you may find it useful : the wall which was meant to separate neighbours brings them together every year in order to repair it. Is it really the borderline?Hope that helps.

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