by Unnikrishnan S on WordPress.org
This plugin can redirect non users or not signed in users to any external or internal url.

Plugin Admin Screen
During development phase you can use this plugin instead of showing \”Under Construction\” page. This plugin will redirect all visitors to a specified url and allow developers or admin to login via wp-admin panel.
Advantages of using this plugin from Under Construction Plugin
1> Your old site is still accessible when your new site is in construction state
2> Admin or developer can login using site-url/wp-login.php
3> Easy to configure
4> Can even redirect to external urls