what is ironic about Tom Sawyer's agreeing to help Huck free Jim? -chapter 33

The irony is in the fact that Tom considers it wrong to do so.

Tom isn't so altruistic as HUck is. Tom does not share a bond with Jim, or any other black person, as Huck does. Tom is in it for the thrill and excitement of freeing Jim. Tom wants to put one of his convoluted plans into action. He also likes the sense of rebellion.

The fact that Tom knows Jim is already free is ironic, especially since Jim doesn't know himself.

Hopefully, I haven't ruined that fact for you.......... it actually doesn't come up until a bit later, and I don't know how far you've been expected to read up until this point.