This stone fort is built with no mortar at all, and still stands. The area also has standing stones, and burial mounds.
Soon the forms become familiar. See prehistoric structures at http://www.irishmegaliths.org.uk/seanchlocha7.htm/ There are fine photos and history here.
This fort is round,with a large circle of high walls and walkways, and rooms within the walls, space for villages and livestock to find refuge from invaders, coming up from the sea in the distance. See Ireland's archeology at http://www.mockingbird.creighton.edu/english/micsun/irishresources/archaeol.htm.