Stone forts are a common element of the archaeological remains in the west of Ireland. Most are equivalent to the earthen ringforts found throughout the country and were probably built as homesteads during the period 500 - 800AD - these stone ringforts are called cashels. However a number of stone forts stand out from the remainder either because of their large size or their prominent locations or because they have complex or massive defensive features. The Western Stone Fort Project is looking in detail at this particular group of large stone forts. Around twenty-five of these forts are known - Dún Aonghasa on Inis Mór in the Aran Islands, Grianán Aileach in Co. Donegal and Staigue in Co. Kerry are the best known examples. The area enclosed at these forts varies from 14 acres at Dún Aonghasa to under half an acre at many of the smaller forts. Some forts may have as many as three enclosing walls; the inner wall is usually the most massive and examples up to 8.50m in thickness and over 4m in height are known.

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. Four forts 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 and which must have been a formidable obstacle for any potentail attackers.

At present we know little about the people who built these forts and the social or environmental conditions which prompted the construction of such large scale defensive monuments. Architecturally some of the Irish sites are broadly comparable to Iron Age (500BC - 500AD) stone forts along the Atlantic seaboard of Western Europe. The presence of a chevaux de frise for example suggests links with stone forts in Northern Spain, Portugal, Scotland and Wales where this feature also occurs. Whether these links represent the spread of ideas, or the movement of people along the Atlantic seaway is a matter of opinion at present but some scholars would look to this route as the possible artery along which Celtic influences first reached Ireland. As part of the project these possible connections are being examined more closely.

Detailed survey of the Irish forts suggests that many are multiperiod sites i.e. what we see today at forts such as Dún Aonghasa and Dún Chonchúir on the Aran Islands and Grianán Aileach in Co. Donegal is a result of remodelling and rebuilding over a long period of time. The stone fort at Grianán Aileach stands within a hillfort which is of prehistoric date. About 80 hillforts or hilltop enclosures have been recorded in Ireland and their construction may reflect a period of unrest marked by increasing delineation of tribal boundaries. Archaeological excavations at hillforts such as Rathgall Co. Wicklow, Haughey's Fort Co. Armagh and Dún Aonghasa on Inis Mór indicate that hillforts were probably occupied by a small elite group of people but that they had very broad social and ritual functions relating to the wider community.

Some of the Late Bronze Age hillforts (built circa 1000BC) were abandoned at an early stage but a small number (e.g. Dún Aonghasa) were substantially remodelled during the following Iron Age - it is also possible that some hillforts were built during the Iron Age but archaeologists do not have enough information at present to be certain of this. Neither can they be sure as to what function the surviving hillforts had during this period - it is possible for example that they retained some ceremonial or administrative functions. A number of sites were probably reinvested as royal centres in the Early Historic period. Grianán Aileach in Co. Donegal is a good example of this - the central stone fort was probably built in the later part of the first millenium AD and was the royal seat of the Cenél n Eogain (Northern Uí Néill) The Grianán of Aileach – a note on its identification. It seems likely that many of the smaller forts included in the study were also royal seats built in the period 600 - 800AD - today these represent the best examples of non-ecclesiastical monumental architecture which survives in Ireland from the pre-Norman period