admin_ability.rb

class AdminAbility
  include CanCan::Ability

  def initialize(user)
    can :access, :dashboards
  end
end

admin_controller.rb

class AdminController < ApplicationController
  before_filter :authenticate_user!
  # before_filter do 
  #   flash[:error] = "Unauthorised Access Denied!" unless current_user && current_user.admin?
  #   redirect_to '/' unless current_user && current_user.admin?
  # end

  enable_authorization
  # rescue_from CanCan::Unauthorized do |exception|
  #   redirect_to root_url, :alert => exception.message
  # end
     
  def current_ability
    @current_ability ||= AdminAbility.new(current_user)
  end
end

error.md

Visiting /admin

CanCan::Unauthorized in Admin::DashboardController#index
You are not authorized to access this page.

routes.rb

  namespace :admin do
    root :to => 'dashboard#index'

Note that DashboardController inherits from the AdminController as class Admin::DashboardController < AdminController

Try can :access, :dashboard instead to match the name of the controller. This is just off the top of my head, haven't experimented with this much recently.