This spectacular fort can be seen from the sea as one approaches Inis Meáin - it lies about halfway along the island and overlooks a natural fault. The fort was defended by a massive inner wall, over 8m wide in places and which enclosed an area 70m by 40m. A slighter outer wall runs concentrically around all but the western side of the fort and a square forework projects from this on the northeast side. The western perimeter was protected by a rock cut ditch, the excavation of which probably provided much of the stone for the construction of the fort itself - 14,000 tons of stone were needed to build the inner wall alone.

The large size of the enclosure at Dún Chonchúir and its prominent and strategic location suggest that it is of prehistoric date. Only excavation could tell us for certain whether it was originally built during the Bronze Age or the Iron Age - like Dún Aonghasa it may have been rebuilt or remodelled on a number of occasions. A number of artefacts from the fort and the surviving house remains (some of square or rectangular plan) indicate that the fort continued to be occupied down into the Early Medieval period. The fort may have been slightly enlarged during this period - at the same time the inner wall was probably strengthened. The forework on the north-east side may have been built then or slightly later in order to protect the entrance.