I have to agree with Ryaner below about this being one of the most impressive sites in the Burren.
It is similar to Dun Aeognus on the Aran Islands in that it uses a cliff face as a natural defence to the north.
Here is a link to an aerial view on Bing Maps to get a better idea of its plan binged.it/13C2kNd
I seem to recall that the valley to the north of it acted as an east – west transport link across the Burren so this would have been a controlling point on this route.