Twenty metres across, built from earth and stone, and now heavily overgrown, it is a crannog, the kind of artificial island dwelling that was constructed throughout Ireland and Scotland from the Bronze Age well into the early medieval period. Builders would pile timber, stone, brushwood, and peat into shallow lake water, driving wooden stakes around the perimeter to hold the mass together and keep the structure from spreading. At Castlefore Lough, those original stakes are still intermittently visible, poking through or just beneath the waterline around the island's edge.

Castlefore Lough itself is a compact, roughly triangular body of water, running about 800 metres east to west and 450 metres at its widest north to south. The crannog sits off-centre within it, positioned close enough to shore that a short boat crossing would have connected its inhabitants to the mainland, but far enough out to offer a real defensive advantage. That combination of accessibility and security was the point. Crannogs served as farmsteads, refuges, and occasionally as the residences of local lords across many centuries of Irish life, and their artificial nature meant that the people who built them had deliberate control over who could approach. The site at Leamanish is documented in Michael J. Moore's Archaeological Inventory of County Leitrim, published in 2003, which remains the principal catalogue of such monuments across the county.