How does Mark Twain use irony in Huck Finn to demonstrate how people
are racist, especially in terms of moral or religious nature in the
particular scene when Huck is shot and Jim appears to help the doctor
and after leaving the island and Jim is harrassed by the public. Thank
you very much.

This article may be of help:
http://www.pbs.org/newshour/authors_corner/jan-june97/fishkin_4-11.html