Walking along the Kerry way you will see a number of circular enclosures of varying size with concentric ditches. Many of these enclosures fall into the broad category of Ringfort. Ringforts are circular sites that were mostly built during the Bronze age up to about the year 1000. They are found throughout Northern Europe. Ringforts come in many sizes and may be made of stone or earth. Earthen ringforts would have been marked by a bank and ditch, often with a stake-wall. Both stone and earthen ringforts would generally have had at least one building inside.
There are many irish language terms for these enclosures which you will see in place names today- ráth and lios was an earthen ringfort; the ráth being the enclosing bank and the lios being the open space within. The caiseal and cathair was a stone ringfort. The term dún was usually used for any stronghold of importance, which may or may not be ring-shaped.