¹Haverty, Martin. The Aran Isles: or, A report of the excursion of the Ethnological Section of the British Association from Dublin to the western islands of Aran, in September, 1857. p. 28. Privately published in 1859. Available online.

²Dalton, John P. "Who Built Dun Aengus? (Continued)." Journal of the Galway Archaeological and Historical Society 14.3/4 (1929): 110.
The author suggests that King Aengus made a living as a pirate: "…unless they still resorted periodically to sea-raiding and smuggling. Aenghus could have kept up but the very poorest semblance of a royal court at Dun Aengus."
Another possibility, reported by Westropp (quoting Edward Ledwich, 1790) is that Dun Aengusa was named much later, after an entirely different Aenghus, one who was King of Cashel, c. 460 CE.

³"Lebor Gabala Pt. 3." AKA Mary Jones. Web. 04 June 2011. <http://www.maryjones.us/ctexts/lebor3.html>.
The full text may be read at this web site.

⁴Cotter, Claire. Dún Aonghasa: The guidebook. Dublin, Ireland: The Discovery Programme, 2014. p. 49.

⁵Haddon, A. C. and C. R. Browne. "The Ethnography of the Aran Islands, County Galway." Proceedings of the Royal Irish Academy, V. 2 (1891-1893). p. 780.
The authors noted, "t is not probable that the O'Briens or the O'Flaherties were Firbolgs. We know that garrisons were several times quartered on the islands, more particularly on Aranmore, and it is not improbable that owing to wrecks and to possible occasional immigrants from Galway of 'foreigners,' that mixture of blood may have occurred during the lapse of the last 500 years.'
They also noted (p. 799), “The population seems on the whole to be an unusually healthy one. Idiocy and imbecility are not common, there being but two cases on the islands, both imbeciles, but possessed of a certain amount of shrewdness.”

⁶Westropp, 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): 34.
He writes, "An unrestored fort is its own record; but, to one who recalls the weird chaos of ruin-heaps in 1878, and contrasts it with the neat, level-topped enclosures left by the restorers six years later, the old descriptions, no matter how rude, assume a great importance, and should be laid before one's readers." Westropp continues on p. 45: "The unnecessary rebuilding and levelling up of parts of the walls and the "tidy" and new appearance thereby produced, show how desirable it was that the work should have been constantly under the supervision and direction of an antiquary who had studied our ring-walls carefully. Left to non-antiquaries and the natives, the work was of course done unsympathetically, like repairing a fence…"
It seems that the only documentation from the 1884-6 restoration effort was the costs ledger. The restoration was perhaps inspired by the romanticism of the Celtic Revival era, as well as the 1857 visit of the British Association.

⁷Harbison, Peter. Review of "The Western Stone Forts Project. Excavations at Dún Aonghasa and Dún Eoghanachta." Journal of Irish Archaeology, vol. 21, 2012, pp. 157–158.

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.)

⁹Cotter, Claire. Dún Aonghasa: The guidebook. Dublin, Ireland: The Discovery Programme, 2014. p. 55.

¹⁰Norman, Edward. The Early Development of Irish Society the Evidence of Aerial Photography. Cambridge: University, 1969. 81-82.
Also: Long, Harry, and Etienne Rynne. "Dún Aonghasa." Journal of the Galway Archaeological and Historical Society 44 (1992): 21.

¹¹Excavations.ie. Searchable Database of Irish Excavation Reports. Web. 05 June 2011.1992 Report. 1993 Report. 1994 Report. 1995 Report.

¹²Long, Harry, and Etienne Rynne. "Dún Aonghasa." Journal of the Galway Archaeological and Historical Society 44 (1992): 11.
The authors on p. 17 suggest that Dun Aengus and its chevaux-de-frise play an important role in discussions of the origin of Celtic groups in Ireland. "The question of when and by what routes Celtic-speaking peoples first arrived in Ireland is fraught with controversy and doubt. The stone chevaux-de frise at Dún Aonghasa is seen as evidence of the influx from Iberia of people speaking Q-Celtic in the wake of the Roman conquest of 133 B.C.33. Some philologists, however, associate the Fir Bolg of Ireland with the Belgae of Belgium and France, who may have occupied sites where, earlier, wooded chevaux-de-frise have been found. Dún Aonghasa is thus at the centre of a debate in which the chevaux-de-frise is used to argue two different opinions."
One of the other three examples of chevaux-de-frise is also on the island of Inishmore, at Dun Dúbhchathair, the Black Fort. There is a virtual-reality view of this fort on the Dun Aengus page.

¹³Westropp, 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):21-22.

¹⁴Westropp, 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): 34.
In addition to Westropp's work in 1909, the most significant of the other investigators were: Roderick O'Flaherty (1684-6) "Ogygia"; Edward Ledwich (1797). "He gives a delusive view...regards the fort as a mandra or monastic enclosure"; John O'Flaherty (1824); George Petrie (1821 and 1857); John O'Donovan (1839); Samuel Ferguson (1853); John Windele (ante 1854); Lady Ferguson (1867). "The Irish before the Conquest"; and Lord Dunraven (ante 1875). He took photographs before the restoration.

¹⁵Petrie, George, and D. J. S. O'Malley. "Aspects of George Petrie. V. An Essay on Military Architecture in Ireland Previous to the English Invasion." Proceedings of the Royal Irish Academy Section C: Archaeology, Celtic Studies, History, Linguistics, Literature 72 (Read, 1834. Published, 1972): 247-48, 266-68.

¹⁶O'Flaherty, John T. "A Sketch of the History and Antiquities of the Southern Islands of Aran, Lying off the West Coast of Ireland; with Observations on the Religion of the Celtic Nations, Pagan Monuments of the Early Irish, Druidic Rites, &c." The Transactions of the Royal Irish Academy 14 (1825): 97-98.

¹⁷Evans-Wentz, W. Y. The Fairy-faith in Celtic Countries. London: H. Frowde, 1911. 416.
As Petrie noted perspicaciously 77 years earlier: "The Antiquities of Ireland have already attracted the attention of several learned men, but the antiquarian knowledge of those persons was confined to literature—they had no general or accurate acquaintance with the ancient remains of our own and other countries. It was therefore but natural that their labors whether guided by a Spirit of rational enquiry, or led on by visionary national predilection, should have almost equally tended to darken rather than elucidate the subjects of their investigation."

¹⁸Westropp, 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): 1-2.
He writes, "Of all the early forts of Ireland we may say that only one has appealed to the imagination, and even to the affection, of the nation, as a building, and become, with most antiquaries, the type and symbol of the countless similar structures, all subordinate to it in interest. At Emania and Tara it is the sentiment and tradition, not the remains, that so appeal ; but at Dun Aengusa the site and the building affect even the coolest mind as no place of mythic or historic association could do."

¹⁹Wakeman, William F. "Aran – Pagan and Christian. Part I." Duffy's Hibernian Magazine 1. January-June (1862): 470.
This article may be read in its entirety here.

²⁰Westropp, 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): 36.

²¹Muldoon, Michael. AboutAran.com “Stories and Photographs from the Aran Islands, Galway Bay, Ireland.” https://www.aboutaran.com/2020/02/dun-aengus-banquet-in-1857-visitors.html

²²Haverty, Martin. The Aran Isles: or, A report of the excursion of the Ethnological section of the British association from Dublin to the western islands of Aran, in September, 1857. p. 20. Privately published in 1859. Available online.

²³Gannon, J.B. "The Unveiled Aran." The Irish Monthly 73.870 (1945): 519-22.

²⁴Grover-Rogoff, Jay. "Dun Aengus." The Hudson Review 38.1 (1985): 83.