The promontory fort on Inis Mhic Aoibhleáin uses the island's own dramatic geography as its primary boundary, with sheer sea-cliffs dropping away on the southern, western, northern, and eastern sides of its northwest portion. A promontory fort is exactly what it sounds like: a defensive enclosure where a headland or coastal outcrop is cut off from the landward side by one or more constructed banks or ditches, leaving the water to guard the rest of the perimeter. Here, the builders had an unusually generous natural advantage.

What human effort was required focused on the southeast approach, the one side the cliffs could not cover. Two earthen banks were raised there to close off the enclosed area, with a further two banks recorded along the northwest and northeast sides of the island. Within the fort there is also a separate enclosure, suggesting that the interior was subdivided or that activity on the site developed in more than one phase. The irregularly shaped landmass seems to have been read carefully by whoever chose this location, with the fort's footprint adjusted to follow the natural contours rather than impose a regular geometric form on them. These kinds of coastal promontory forts are found at various points around the Irish coastline and are generally associated with the Iron Age, though precise dating for individual sites is often difficult without excavation.