http://www.museumsofmayo.com/ceide-fields/img/neolithic-field-wall.jpg those are the rubble of the walls that have long been uncovered. Most of them are still intact under the peat up to a meter or more. I can't find a photo, but in one of neil Oliver's documentaries, the history of ancient Britain (age of farming I think ita called on youtube) he exposes an intact wall. They cover a truly expansive area, all under the peat now.