# all of this is in /app/controllers/

# file: application_controller.rb
class ApplicationController < ActionController::Base
#...
end

# file: admin_controller.rb
class AdminController < ApplicationController
  before_filter :auth_required

  def auth_required
    #...
  end
end

# file: admin/something_controller.rb
class Admin::SomethingController < AdminController
#...
end