Creggankeel is a small, partly ruined stone ringfort on the central plateau of the island, a few minutes' walk inland from the pier. The circular wall is broken down to roughly chest height on most sides, but the outline is clear and the interior is large enough to make out where buildings once stood.
There is no signage and the fort sits in an unkempt field surrounded by the usual maze of dry-stone walls — locals can point you towards it. It is one of dozens of such cashels scattered across the Aran Islands, dating from around the early medieval period when these defensive enclosures dotted the western Irish coast. National Monument designation; access is informal.