Even if you cover up the brick with drywall, it will 'protrude' from the rest of the wall. (I'm assuming the brick is decorative AND it's not structurally holding up the floor above. So, ...) Personally, I'd take it out since it is decorative, and make the wall flush all the way. To do what you want to do, you'd have to 'box-in' the brick and drywall around it. So, you'll lose about 4" on either side of the fireplace to the 'box' and the drywall. Stud around the brick as if it was a wall (which it is.) Put the studs 16" OC.