You can get the IP address from $_SERVER['REMOTE_ADDR'] (though it won't hide someone using a proxy, if that worries you: http://roshanbh.com.np/2007/12/getting-real-ip-address-in-php.html
Anyway, using the IP from that and a TIMESTAMP column on your database http://dev.mysql.com/doc/refman/5.0/en/timestamp.html you can store the ID of the Admin from the admin table and have a standard auto-incrementing ID as the primary key of your new 'admin_log' table on the database.
There for you can just retrieve the last record from the file for a given Admin to show then they last logged on and the insert a new record on the table for this time they're logging on.
Alternatively you can add a TIMESTAMP column and a column to store the IP on the Admin table and when someone firsts logs in you retrieve the details to display to them and then update the admin database with the current IP address (forcing the TIMESTAMP to be updated in the process).