application_controller.rb

class Admins::ApplicationController < ApplicationController
  layout 'layouts/admins'
  before_action :authenticate_admin!

  # def authenticate_admin!
  #   return render_404 unless current_user.is_admin?
  # end
end

home_admin_controller.rb

class Admins::HomeAdminController < Admin::ApplicationController
  def index
  end
end

routes.rb

Rails.application.routes.draw do
  namespace :admins do
    root :to => 'home_admin#index'
    resources :admins
  end

  root 'home#index'

  devise_for :admins
  devise_for :users
end