How do I attach the rock to plastered interior chimney (brick)?

You can use construction adhesive and some pressure against it till it dries.

If you want to fur it out for some reason(ie running wires, or removal/maintenance if damaged in the future) anchor a furring strip or 2x4 to the brick and attaching drywall like normal.


Ken

KBC is on the right track, you can also use 3/4 treated 1x3 and attach to the brick, and attach the sheetrock to that, keep in mind there are setbacks to keep combustable material away from the firebox