Do I have studs on my plaster and lath wall around/above fireplace?
It sounds like you have some arrangement of thin furring strips that has been adhered to the original fireplace in order to install some sort of backer board or drywall. If so, these furring strips might be enough to hold your TV but would be safer to take a look. It's unlikely there are studs around a fireplace in a house that old. I'd assume the original brick is still behind all of that.