I think from what you are describing you will need to stick a small strip of plaster over the end of the bricks and finish off with an external corner.
See here how to install externals
http://www.how2plaster.com/angles.html
If you didnt want to return the plasterboard around the end of the brick wall, then you should use a casing bead, stopping bead, or stopping angle to finish off the end of the sheets.
Casing bead is sometimes called capping bead, it comes in metal or plastic. It is is not perforated therefore is not stopped up, when painted you will see the edge of the bead.
Stopping bead is similar to casing bead but is perforated and is stopped up giving you a smooth finish.
Stopping angle is similar to stopping bead in the way that it is also perforated and is stopped up. The difference stooping angle is L shaped and can be put on after the sheets are hung.
While both stopping bead and casing bead are U shaped and in most circumstances needs to be slipped onto the sheet before its hung.
Hope this helps.
Cheers