8Westropp, T.J. "A Study of the Fort of Dun Aengusa in Inishmore, Aran Isles, Galway Bay: Its Plan, Growth, and Records." Proceedings of the Royal Irish Academy Section C: Archaeology, Celtic Studies, History, Linguistics, Literature 28 (1910): 12.
Westropp's suggestion regarding the Inner Enclosure's original shape stems from his reading of the 1684 description of Dun Aengus by Roderic O'Flaherty (1629-1718), Ogygia, or, a chronological account of Irish events: collected from very ancient documents.
"Where, or how far, the sea had cut into the hill when these works were built. O'Flaherty seems to imply that the middle ring was entire, like " the bawn of a castle," in 1684 ; but it then stood " on the brim of a high cliff."1 Probably the old second wall was originally entire ; but this is uncertain, for the stone fort of Cahercommaun, in Clare, on the edge of the Corcomroes, has a central ring and two crescent walls.2 It is strikingly like Dun Aengusa. The central fort is even more massive ; but it overhangs a dry valley rising at both ends, so is evidently in its original condition so far as regards the plan. Another alternative is possible, namely, that (as at Dun Conor and the Clare forts of Caherlisaniska and Langough) the central fort was a ring with the outer enclosures looping in to meet its wall. We have, however, only found this looping in forts on flat fields and low ridges?never at high cliffs or slopes, or even on a low shore when there is deep water beyond it, as at the crescent fort at Cahernacalla on Ballycar Lake.3 The crescent wall/ therefore, does not necessarily prove a fall of the cliff, for it is common inland in Ireland, and indeed all over central Europe and in America."
Clair Cotter, the modern excavator of the site, writes, "A number of factors colour the overall picture of the site presented here. First, part of the monument has almost certainly fallen into the sea. The most convincing evidence for this is provided by the fact that structure 9 in the inner enclosure and structure 3 in the middle enclosure are both located at a distance of only 3m from the present cliff edge (Fig. 4.1). These appear to have been houses and almost certainly had organic superstructures. Even if there was a wall along the cliff edge during the occupation of the site, it seems highly unlikely that any timber-built structure would survive even a summer season in such an exposed location. It is impossible now from a geological perspective to quantify just how much of the site may have disappeared since the hilltop was first enclosed during the late Bronze Age, or in the thousand years or so since it was abandoned." (Cotter, Claire. The Western Stone Forts Project: Excavations at Dún Aonghasa and Dún Eoghanachta . Vol. 1. Wordwell, 2015, p. 59.)