DRY approach with different request type

You're better off extending the controllers from a "base" one which has this check in it.

http://www.codeigniter.com/userguide3/ge...core-class

PHP Code:

class Welcome extends MY_Controller {

        public function __construct()
        {
                parent::__construct();
        }

        public function index()
        {
                $this->load->view('welcome_message');
        }
} 


Then in MY_Controller you can put your check code in the constructor and redirect if it fails, as you said.