Добавьте контроллер приложения в ваше пространство имен и фильтр до него.
class ApplicationController < ActionController::Base
end
class Admin::ApplicationController < ApplicationController
# these goes in your namespace admin folder
before_filter :check_authorized
def check_authorized
redirect_to root_path unless can? :admin, :all
end
end
class SomeadminController < Admin::ApplicationController
def some_action
# do_stuff
end
end