I'd recommend taking advantage of the isAuthorized() function that you can add in the controller, or the model. Set the AuthComponent::authorize = {'controller'|'model'} to choose which you want to use.
Then you write an isAuthorized() function in the model|controller that returns t/f on auth/not auth for each action. You can do some row-level checking as well, if you'd like.
Now, if instead you just wanted to redirect an admin to their correct pages on login/etc, you can add code to the beforeFilter() method (either in a specific controller, or in app_controller.php). In that, just check to see if the admin value set by the app is the same as the user's admin value (which will be stored by AuthComponent in the Session data, accessible by $this->Auth->User()). Then route appropriately to the admin/non admin areas.