As fireclay is often the seatearth to coal, a fireclay miner was usually the same thing as a coal miner.
I wonder if it was a term for any non-coal miner? i.e. anyone who wasn't coming home black at the end of the day. Or, by the same token, a surface worker at a coal mine.
As has been said, the only other extraction in the area would have been stone. Though I'm not sure there was any underground stone extraction within walking distance.