How do I protect the walls next to a 24 foot tall red brick fireplace when painting it?
It makes me die a little inside when people paint over raw red brick....

That said, you tape and paper the walls where they meet the brick. Then you plastic from the paper as far out as you think you might overspray. You may even want to form a "curtain" out of the plastic to keep the brick in a make-shift paint booth. 

It is just as you would mask any other wall, though you will likely need to touch-up the seam where the brick meets the treated wall, it shouldn't be too bad.