Getting the absolute path of the controller

Does anyone know how I can determine the absolute path of a controller? I'm using the controller's absolute path as a key for my site's security, but I'm coming across an issue where the form_remote_tag method is putting in the relative path of the controller, not the absolute path. It's a case where I can't explicitly pass in the controller path myself.

What do you mean by “absolute path”? File system path? full URL path?