What is ironic about his dream?

David dreams of a mutant calf being purified at a ceremony but instead the calf turns into Sophie. There is some light irony here but not too much.

The irony in his dream has religious connotations. In it, Sophie begs the people around her for help, but nobody helps her; they just continue to sing a "hymn" to God. The irony in this is that God is all-loving and merciful; he loves all the creatures of the Earth, and yet the people who stand by and watch are anything but what God stands for.

Well, that's the irony through this whole book. I think it's a theme more than anything else. I'd be reluctant to specify that particular scene as overtly ironic.