The view from the summit of the fort is most impressive and solemn: the desolate-looking fields…fall away to the golden crescent of Kilmurvey strand, and rise up the opposite hill…to the old lighthouse near Dun Oghil. Eastwards runs the long range of steep, dark headlands, and deep bays, rarely unsheeted by high-leaping spray…The limits of the view on clear days reach from the giant peaks of Corcaguiny in Kerry to those of Connemara; while to the south-west is only the horizon of the landless deep, whirling sea-birds, and the sparkling silver tideways. –T. J. Westropp, 1910
As the legends recounted in the Lebor Gabála (Book of Invasions) would have it, the mythological race of the Fir Bolgs built Dun Aengus after they sought refuge on the island in the first century CE. They named the fort after their chieftain, Aenghus, King of the Clann Umoir, “the sterling pilot of his people who lives in legend as the founder and first lord of Dun Aengus.”
The Fir Bolg gave them [the Tuatha Dé Danann] battle upon Mag Tuired; they were a long time fighting that battle. At last it broke against the Fir Bolg, and the slaughter pressed northward, and a hundred thousand of them were slain…The Fir Bolg fell in that battle all but a few, and they went out of Ireland in flight from the Tuatha Dé Danann…Thereafter they came in flight before Cairbre under the protection of Meldb and Ailill, and these gave them lands. This is the wandering of the sons of Umor. Oengus son of Umor was king over them in the east, and from them are named those territories, Loch CIme from Cime Four-Heads son of Umor, the Point of Taman in Medraige from Taman son of Umor, the Fort of Oengus [Dun Aengus] in Ara from Oengus, the Stone-heap of Conall in Aidne from Conall, Mag Adair from Adar, Mag Asail from Asal in Mumu also. Menn son of Umor was the poet. They were in fortresses and in islands of the sea around Ireland in that wise, till Cu Chulaind overwhelmed them.
Lebor Gabála Érenn: The Book of Invasions of Ireland
§39-54: “The Nemedians and the Fir Bolg”