A "bullaun" is a large depression - a massive cup mark often upto 30cm aross and 20cm deep. We tend to count any cup over about 8cm across as a bullaun rather than a cup-mark if it's on a flat horizontal surface. The concept isn't very much considered beyond the Irish shores, but as we increasingly see they are much more widely spread. I know of them in Spain, France, England, Wales, Scotland, Lithuania, Gotland and a possible on Malta.

A "bullaun stone" is a stone with bullauns carved into it.

Hope that helps.