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?