Stone forts are a common archaeological feature in the west of Ireland. The Irish words caiseal (English ‘cashel’), cathair (‘caher’) and dún (‘doon’), all meaning ‘a fort’, are used to describe them. Many appear to be similar in function to the earthen ringforts found throughout the country and were probably built as homesteads during the period 500 - 800AD. A number of stone forts stand out due to their size, their prominent locations or their complex and massive defensive features.
Twenty-five of these complex forts are known, among them Dún Aonghasa on Inis Mór in the Aran Islands, Co. Galway, Grianán Aileach, Co. Donegal and Staigue Fort, Co. Kerry. Some forts may have as many as three enclosing walls, the inner wall being the most massive. Apart from their strong defences these forts share common architectural features, such as terracing of the walls, stone steps, and passages or chambers within the enclosing walls.
A small number have the unusual feature of a chevaux de frise - a band of closely set upright pillars of stone which formed an extra line of defence. The most spectacular example of a cheveaux de frise in Europe survives at Dún Aonghasa. Some forts date as far back as the Late Bronze Age (circa 1000BC) and a small number were substantially remodelled even into the early medieval period. Grianán Aileach, Co. Donegal provides a good example of this phenomenon. The central stone fort was probably built in the later part of the first millenium AD and was the royal seat of the kings of Cenél nÉogain (Northern Uí Néill).