That small hollow is, in archaeological terms, a question rather than an answer: it may mark the roof of a souterrain that has quietly caved in on itself over the centuries.

Souterrains are underground stone-lined passages or chambers, typically built during the early medieval period in Ireland, probably used for cool storage and possibly as places of refuge. They are often found within ringforts, the circular earthwork enclosures that were the standard farmstead type of early medieval Ireland. When a souterrain's capstones or corbelled roof finally give way, the result is precisely this kind of subtle surface dip, easy to miss and easy to misread. The ringfort at Teereeven in mid Cork is recorded as containing what is tentatively identified as one such collapsed example, its presence suggested by that single, unassuming hollow in the earth.