What is ironic about the notice in the beginning of "The Adventures of Huckleberry Finn?"the notice is in the beginning after the preface.
The irony in the notice, like the previous poster noted, was Twain's asking the reader to not use this story as a way to moralize, and instead to use it to entertain and amuse the reader. The irony may also be that he is basically asking people not to learn from the Adventures of Huck, which are, themselves, learning experiences that build upon Huckleberry and help him understand the world around him.
He also asks people not to find a plot, which is impossible because the story is so well-planned and narrated with such a sequence of events, maybe Twain was congratulating himself ahead of time for a job well done and used irony to disguise it.
PERSONS attempting to find a motive in this narrative will be prosecuted; persons attempting to find a moral in it will be banished; persons attempting to find a plot in it will be shot.
The notice that you are talking about is meant to be a humorous warning. It says, among other things, that people will be shot for trying to find a plot in it.
I would say that the most ironic part of it is the part where Mark Twain tells us that people who try to find a moral in it will be banished. I believe that this sentence is ironic because there are actually quite a few morals that that Twain is trying to convey with his book.
So, by saying that people should not look for a moral, he is being ironic because there really is more than one moral to the story.