It's unclear to me if these two controllers are running on the same machine or not since you referenced DRb. If they are running on the same machine how do you instantiate "AnApplicationController"? That is, Rails instantiates the controller for each incoming request and from your example that would appear to be "AuthController".
Could you simply do AnApplicationController.new.listenForAuthControllerData(some_data)? Of course, this would require setup and tear down for each invocation. Another alternative MIGHT be to instantiate it one time and store it in a class variable. If you have multiple instances of the Ruby interpreter running on your server though that might not work depending on the requirements of your application.
-Paul