It is an artificial island, a crannog, and it has been there long enough to accumulate four metres of height, a thick blanket of soil, and a dense tangle of vegetation that makes it easy to miss if you do not know to look.

Crannogs are lake dwellings, constructed by piling timber, brushwood, stone, and earth into shallow water to create a platform that could be occupied, defended, or used for storage. They appear across Ireland from the Bronze Age through to the early modern period, and many continued in use for centuries, accruing layers of material that can be difficult to unpick without excavation. The Carrownacon example measures roughly 30 metres north to south and 28 metres east to west, with a stone revetment, the reinforced outer facing that helped hold the mound together, running from the west-south-west around to the east. What makes this particular site quietly arresting is what protrudes from the surface: several upright timber posts still mark the mound's eastern and south-western edges, and three timber planks push up through the earth on the south-east side. Wood surviving in waterlogged or semi-waterlogged conditions is archaeologically valuable and relatively uncommon, and here it is simply visible, sticking out of the ground.

The site is accessible by land during the summer months when water levels drop sufficiently to make the approach manageable. It is heavily overgrown, and the soil cover towards the top of the mound is deep, so the structural details, the revetment stones and the protruding timbers, reward a careful look rather than a casual glance from a distance.