This odd structure is behind the church, out of sight from most angles. It seems to be built into the boundary wall and perhaps a cliff face (there's a hill on the other side of the wall, but I don't know whether the wall is set into the hillside), so I'm not sure whether the sealed doorway is the entrance to a tomb or merely a false entrance no deeper than the thickness of the wall. The sandstone in the doorway is different to that in the surround and the wall itself, which could suggest it was a later addition.
It's a pity the plaque is missing; perhaps it's inside the church.