You can buy small square wooden blocks from your local timer yard.
You can fix these to the wall first by using long screws which will go though the plasterboard and into the brick. Then attach the handrail brackets to the blocks with the normal small screws.
If when you say a plasterboard wall you mean a stud wall, then you will need to locate the studs (stud detector) or by taping the plasterboard until you hear a more dense noise. Again I would attach the blocks to the studs then attach the hand rail brackets to the blocks.
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