Re: Painting Brick Fireplace
Interior brick is treated as any other brick/masonry. You will want to use a water soluble primer such as Zinssers 123, followed by an acrylic interior housepaint. If there is significant soot build-up on the brick, you will want to clean the brick with TSP.
What paint sheen to use is a matter of personal taste, however, the more sheen, the more washable a paint tends to be.
If the fireplace insert is able to be loosened or removed, I would take the time to back it off a little from the brick. Otherwise, you can just tape off the insert with blue tape and plastic.
A combination of roller and brush will work well. Use the roller to get most of the paint material up there, and then back brush to force the paint into the mortar seams and indents. Use at least a 1/2 inch roller cover.