People using the workhouse hospital may not have been inmates of the workhouse. Your ancestor may have been brought home to be waked, had a funeral and burial in the family plot.
Even if they were an inmate unless they had no family they would probably be brought home.
Most families didn't have a headstone at the time as they couldn't afford them instead they just used something like a large stone to mark the spot and if later generations were still using the grave they may put up a stone but wouldn't know the names of everyone buried there.
