What makes this poem ironic?
The poem "There Will Come Soft Rains", which is read by the robotic house in the story of the same name, is ironic because it unknowingly comments on the reality of the world in which the house exists.
The poem begins in a pastoral and reflective tone, talking about nature and the beauty of natural things, but ends with the melancholy suggestion that nature will not notice or care if humanity disappeared. In fact, in the world of the story, humanity HAS disappeared, as the robotic house is one of the few surviving structures following a nuclear war, and most if not all people have been dead for years.
Some of the irony also comes from the fact that the house, while complex, is far from intelligent. It is incapable of recognizing that it is reading the poem to an empty room, and that the house's owners were incinerated by nuclear weapons and their remains are scarred into the side of the house itself. The house is incapable of truly noticing or caring that humanity is gone, and as the house is reclaimed by natural forces (fire) that last vestige of humanity goes with it.