One of my clients had the issue of redirection to the parent site from a subdomain when trying to access wp-admin.
Then I realized this is actually default behavior of this plugin.
There's a few ways to deal with this. The first we found was modifying the site_url value in the DB for the subdomain.
The more best-practices way would be to unhook 'dm_redirect_admin' from 'admin_init', perhaps on init.
Perhaps this could be made an option in the Domain Mapping plugin though? So a user can check on/off wp-admin redirecting? Seems like it might be a relatively easy addition, although will probably open up a configuration vs. convention discussion.