From its early beginnings, Dún Aonghasa would have been comparable to some of the high status hillforts on the mainland. These were important places in society, where communal activities would take place like inaugurations or religious ceremonies, and a place where agreements could be made or disputes ruled upon and justice dispensed. Certainly, the establishment of a large fort here would have taken the cooperation and labour of a large number of people, and this hints at a strong social cohesion during the Bronze Age. Interestingly, a considerable amount of evidence for Bronze Age metalworking was discovered on the site during excavations carried out by Claire Cotter and the Discovery Programme. Perhaps at this early period the manufacture of metal tools was akin to sorcery, a secret skill and knowledge reserved only for the select few, to be practised only within certain important, and perhaps sacred, places. Similar evidence was also found at the broadly contemporary Rathgall Hillfort in County Wicklow.
As well as its formidable boundary walls, Dún Aonghasa possessed a further layer of defence – a chevaux de frise that surrounded the middle enclosure of the fort. This chevaux de frise was a band of large stones, some up to 2m (6.5 feet) tall, that were stood upright by being wedged and propped up with other large stones. The tightly packed pillars would have made it difficult for enemy forces to advance in a group or to form a shieldwall on their approach. As the bedrock of the land makes it difficult to dig, this was a very practical and effective alternative to a deep ditch.
During the early medieval period, Dún Aonghasa was transformed with an enormous amount of construction that incorporated the Bronze Age enclosures within massive new drystone walls. These were much higher and broader than the originals, and almost swallowed the older walls within the new fabric. This is clearly seen with the middle enclosure, where the original Bronze Age wall was almost doubled in both width and height. The inner enclosure was also refortified during the early medieval period. At this time, the Aran Islands were a disputed territory, as a frontier point between Connacht and Munster. The redevelopment of the ancient fort was a powerful statement of the dominance and power of the ruling family. Archaeologists also found evidence of early medieval houses within the inner enclosure, along with a brooch-pin, a comb and burials, including the remains of two young men dated to between AD 680–950.