3Dalton, 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.