Dating the forts is difficult, since they have been used and reused so often. It is likely that all seven forts on theisland were in use around 800 CE -- but some date to 1100 BCE. Dun Aengus specifically lseems to have signs of inhabitance from about 1500 BCE to well after 800 CE.
There are a number of other duns on the island -- Dun Eoghanachta is near Kilmurvey, Dun Dúcathair (the black fort) is another promontory fort that is nearly falling into the sea, and Dun Eochal, west of Mainistir, to name a few.
lost in ireland 2005 travelogue and photos © rfingerson