Original very old plaster can dry out completely and crumble. This can also occur when fireplaces are being replaced or when a new wood-burner is installed. 
 
If the old plaster is onto brickwork, it needs to be scraped back and removed completely. A bonding agent is applied onto the brick surface prior to dot and dabbing new plasterboard to level with the existing wall, filling any gaps created on replacing the old fireplace for new.