What is the irony in the poem "There Will Come Soft Rains"?

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. 

Why is the first sentence of "There Will Come Soft Rains" ironic?

The opening line of the story begins with the smart home making an announcement that it is time for the family that lives in the house to wake up. The voice is pleasant-sounding, as we are told that the voice sings its announcement of the time; however, readers are also told that for some reason the voice sounds slightly scared that nobody is present to hear its announcement.

It is an ironic line because it turns out that the house's fears are correctly placed. The family that once occupied this intelligent home has been vaporized from the surface of the planet. Additionally, while the house is smart enough to know what happens for each occupant of the house at specific times, the house does not actually have a way of knowing whether or not the occupants are present in the house. The house goes about its business regardless of the fact that there are no people around to benefit from its efforts. This is hugely important to the piece because it shows that the technology, as well as the nature surrounding the property, would scarcely know if humans were gone or not, and that is how the poem within the story ties into the overall message of this piece.

Not one would mind, neither bird nor tree,
If mankind perished utterly;
And Spring herself, when she woke at dawn
Would scarcely know that we were gone.

Explain the use of irony in "There Will Come Soft Rains".

Irony is crucial to what Bradbury is trying to communicate in this excellent story that presents us with a future world that is so advanced that human beings need do nothing, because technologically advanced robots do everything for them. The most important moment of irony comes when the computer chooses to read the poem that gives the story its title, "There will come soft rains." It is highly ironic that the computer would choose to read a poem about the soft, life-giving rains of spring at a time when the "soft rain" of deadly radioactivity is falling. It is also ironic that the poem should refer to the death of mankind, not knowing that it would actually happen. Note what the poem says:

And not one will know of teh war, not one
Will care at last when it is done.
Not one would mind, neither bird nor tree,
If mankind perished utterly;
And Spring herself, when she woke at dawn
Would scarcely know that we were gone.

Note how the poem explains the natural world continuing unawares, even after a war and the extinction of the human species, much as the workings of the house continue after the family has perished.

Irony, therefore, is central to the story on so many different levels. Humanity is shown to have reached great technological capabilities but also technological advances have resulted in humanity's own extinction, thus presenting us with a severe warning about the dangers of technology.