Not entirely sure what you are asking.. Are you looking to find out what service accounts the mirroring service is using to connect to the various mirroring servers? Look at the principal_name and remote_user_name columns in master.sys.dm_db_mirroring_connections on the principal.
If you are asking what logins are being used to connect to the mirrored databases, you can look in the security folder of the mirrored database to see the ones allowed in or the Activity Monitor to see the ones currently in use.
Shaun