By default the admin access is not enabled on the outside interface. You have to specifically turn that ability on to access it on the I/F. However if you are only using the inside I/F and you want to prevent access to that realm from external IP addresses you could so by using "Authentication Policy"
Simply limit the "source IP" for the specific admin realms to your internal IP addresses.
This really only hides it if you create a large number of new admin urls and then delete all but one of them. This is because the /dana-na/auth/url_[N]/welcome.cgi has no randomization in the URL (url_[N] is the Nth url created.
In fact, if your admin users have bookmarked your sign-in page and you delete and re-add the sign-in policy, their browser bookmarks will no longer be valid.