I guess it's the typical dashboard question, which has been tricky in the past.
The option you are looking for is use_authentication=0 in cgi.cfg - though, this is actually a bad idea given your current setup with authorization by your existing users and such.
There's still another method in achieving that without harming your existing install - create another apache configuration serving your Classic UI below "/icinga-dashboard" and use Satisfy Any for the authorization, e.g. your local network. Though, SetEnvIf for the REMOTE_USER will be required then too, and some more advanced apache knowledge in general (or your preferred webserver).
A more easy method (but a bit more overhead) will be installing the Classic UI as a standalone application, having their own cgis/html and only reading the data source from Icinga Core, serving only what you require.
Since we are using the Classic UI while developing Icinga2 Compatibility Layers, we finally made that happen with Icinga 1.9 release - you can install Classic UI Standalone by following this documentation: https://wiki.icinga.org/display/howtos/Setting+up+Icinga+Classic+UI+Standalone