One of the main points of irony is that the men stay safe in a slaughterhouse, which is usually seen as a place of violent death. The usual place of death is like a womb or a tomb (underground) that keeps the men safe from the destruction that is happening above them.
When the men emerge from the slaughterhouse, Vonnegut describes the landscape as barren, calm and quiet, like the moon. These are descriptions that one wouldn't normally connect with the aftermath of a bombing. Usually we think of destructions and death, blood and screaming, but Vonnegut instead focuses on how quiet and serene it is, and how the men climb over and through the craters left by the bombing.
The last main instance of irony is where the men stay after the bombing. A blind German innkeeper lets them stay in his barn--after their country just committed a horrible atrocity on the nearest town. These men, these soldiers, who were supposedly part of the destruction, are paralleled to Jesus--they hide in an underground tomb, emerge to a different world, and then sleep in a stable.