How can I fix a cabinet to plasterboard with brick wall behind?
I'd recommend these lag shields


 but the info in this link can help you decide. You could probably go with simple lead sleeves but without knowing the condition of your brick or the exact weight of the load you'll be supporting lag shields are a safer bet. As far as going through the plasterboard goes, it's not any different than attaching something to studs through drywall, you just need to make sure you're drywall is compressed between the cabinet back and the supporting wall. If there is a void between the two it will collapse when you tighten down your anchors or worse, when you put weight in your cabinet.