Chad H.
Yelp
Very impressive. Dun Aengus is an ancient fort which apparently used to be a full circle, but is now a semi-circle due to cliff erosion. From the village of Kilronan, there are buses that take you there, or you can rent a bike (like I did) for €10 and bike there. On a nice day it's a very pleasant bike ride, especially if you go along the coast (you can stop halfway and look at the seals!)
Anyway, I found the fort to be fascinating. I love really old structures like that, and it's super cool how you can walk right up to the cliffs. If this was America, there would be ropes and fences and stuff everywhere preventing you from getting near the cliffs, but here they just let you walk wherever. So that's cool. It's only a few euros to get in, so if you've gone all the way to Inishmoor there's basically no reason NOT to go, as its the highlight of the island. There's also a small / lame visitor's center and a little cafe, in case your hungry.
The only note I'd make is that it's a bit of a hike (10-15 min) up a semi-steep hill to the fort. Plan on an hour round trip.