admin-routes.rb

# in routes
  namespace :admin do
    resources :sample, only: :index
  end

# app/controllers/admin/sample_controller.rb
class Admin::SampleController < ApplicationController
  before_filter :authorize_admin

  def index
    # do shit
  end
end

# in app/controllers/application_controller.rb
class ApplicationController < ActionController::Base
  #... other shit above
  #...
  
  
  def authorize_admin
    redirect_to root_url unless current_user and current_user.admin?
  end
end