To use a realm alias today, the admin has to do 2 things:
1. Edit the realm “subrealm" and add a DNS alias (e.g. “subrealm.example.com”)
Then navigate to Configuration -> Servers and Sites -> Your server name -> Advanced
2. Add a property to the property table called com.sun.identity.server.fqdnMap[subrealm.example.com] with the value subrealm.example.com
It would be convenient if the Admin Console performed both actions when realm alias field is set.