If routes file is set to access new url and you want to set redirection for old URLs then Use following code in .htaccess. otherwise let me know in detail what you want to do.
RewriteEngine on
RewriteBase /
RewriteRule /mauritius_holiday_rentals/apartment/$1 /mauritius_holiday_rentals/$1 [R=301,L]
Routes.php config file code
$route['mauritius_holiday_rentals/(:any)']="mauritius_holiday_rentals/apartment/$1";
Let me know if any problem.