if (! $this->session->userdata('first_name'))
    {
    redirect('login');
}

<?php
class Current_User {

    private static $user;

    private function __construct() {}

    public static function user() {

        if(!isset(self::$user)) {

            $CI =& get_instance();
            $CI->load->library('session');

            if (!$user_id = $CI->session->userdata('user_id')) {
                return FALSE;
            }

            if (!$u = Doctrine::getTable('User')->find($user_id)) {
                return FALSE;
            }

            self::$user = $u;
        }

        return self::$user;
    }


    public static function login($email, $password) {

        // get User object by email
        if ($u = Doctrine::getTable('User')->findOneByEmail($email)) {


            // to ge the mutated version of the input password
            $u_input = new User();
            $u_input->password = $password;

            // password match
            if ($u->password == $u_input->password) {


                $CI =& get_instance();
                $CI->load->library('session');
                $CI->session->set_userdata('user_id',$u->id);
                $CI->session->set_userdata('username',$u->username);
                $CI->session->set_userdata('first_name',$u->first_name);
                $CI->session->set_userdata('last_name',$u->last_name);

                self::$user = $u;

                return TRUE;
            }

            unset($u_input);
        }

        // login failed
        return FALSE;

    }


    public function __clone() {
        trigger_error('No duplicates allowed.', E_USER_ERROR);
    }

}

$CI->session->set_userdata('logged_in', 'TRUE');

if (! $this->session->userdata('logged_in')==TRUE)
{
redirect('login');

<?php
class MY_Controller extends Controller
{
    function __construct()
    {
        parent::Controller();
        if (! $this->session->userdata('first_name'))
        {
            redirect('login'); // the user is not logged in, redirect them!
        }
    }
}

class Secret_page extends MY_Controller {

 // your logged in specific controller code
}

/*
| -------------------------------------------------------------------
|  Native Auto-load
| -------------------------------------------------------------------
|
| Nothing to do with cnfig/autoload.php, this allows PHP autoload to work
| for base controllers and some third-party libraries.
|
*/
function __autoload($class)
{
    if(strpos($class, 'CI_') !== 0)
    {
        @include_once( APPPATH . 'core/'. $class . EXT );
    }
}

Admin_Controller.php
MY_Controller.php
Public_Controller.php

$user_session = $this->model_user_lite->Session_Check();
    if ( $user_session == FALSE )
    {
        redirect('main/about');
        exit();
    }
    $data['auth'] = $user_session;
    $user_id = $this->model_user_lite->Session_UserID ();

function Session_Check ( $return_link = FALSE )
{

    $auth   = $this->session->userdata('logged_in');
    if ( $auth == FALSE )
    {
        $return_value = FALSE;
    }
    else
    {
        $return_value = TRUE;
    }

    return $return_value;

}