You’ll need to drop the database, and restore the database again to get mirroring working.
Drop all the logins as well before restoring the database.
Go to MS KB 918992 and install sp_help_revlogin on the current system. Then run sp_help_revlogin. This will give you the SQL Scripts needed to create the logins using the same SIDs as were used on the original system. You can then restore the database, and ship the logs from the source to the new server, then setup mirroring as normal.