They might have served as boot camps for Sweyn Forkbeard's men before his invasion of England in 1013.
The debate on chronology is primarily a result of the huge number of ringforts and the failure of any other form of settlement site to survive to modern times in any great quantity from the period before the Early Christian period or from Gaelic Ireland after the Anglo-Norman arrival.
is based on the absence of any other settlement form of appropriate date in those landscapes.
In other words, if the Gaelic-Irish did not live in ringforts, where did they live?
If one were to accept a defensive function for ringforts, it would seem that after the introduction of more complex forms of defensive structures into Ireland this would naturally lead to the use of ringforts and raised raths in a manner analogous to the contemporary Norman buildings.
While it would seem probable that some ringforts may have seen continuation in the Later Medieval period as adapted or imitation mottes it seems doubtful if the continuation that ringforts were still being built on a more general scale throughout the country, and the evidence put forward for such a theory would appear quite slim.
Indeed, in a number of cases it would appear that either the Normans converted existing ringforts into the basis of the future construction of mottes and earthworks, or that the Gaelic Irish, through the use of raised raths, sought to emulate the Norman example.
Some L Plan Castles, such as Balingarry Castle in Ireland originated as ringforts.
In Cornwall There are seven known ring fortresses in Denmark and southern Sweden, all from around 980 in the Viking Age.
As only a small portion of ringforts have undergone total excavation, and the fact that these excavations have not taken place on anything like a national level, the evidence is insufficient to place all ringforts and the origins of them within the Early Christian period.
Limbert argues instead, that the ringfort should be seen in the context of a variety of similar developments in Britain and the European Continent, particularly in Iberia and Gaul.
In his work The Irish Ringfort, Stout has sought to use the radiocarbon and dendrochronological dates from 114 ringforts and associated sites to find an overall date pattern for the use of ringforts; and through this has placed over half of all ringforts in the period 540 AD to 884 AD with two-thirds falling within the 600 AD to 900 AD period.
While this method has brought the dating of the ringfort phase of Irish history to an ever more accurate level, certain problems do exist with his analysis.
Currently, excavations are ongoing at Sandby borg, which was the site of a massacre in the 5th century A. It is also possible that the Hill of Tara is an early type of ringfort.