It is roughly fourteen by sixteen metres across and barely rises above the surface, yet it is almost certainly one of the most significant medieval sites in the county. This is a crannog, an artificial or partly artificial island constructed from timber, stone, and compacted earth, and used throughout Irish prehistory and the early medieval period as a defended dwelling place. The oak piles still visible on the northern side, the boulder matrix exposed by erosion on the shore, and the submerged kerb of stones just beneath the waterline suggest a structure that was carefully engineered, even if it now appears as little more than a reedy hump.

The lake itself was recorded as Spark's Lake on Ordnance Survey maps of 1834, though it is now known as Convent Lake. A map drawn in 1591 shows Macmahoon's house on an island here, placing the Mac Mahon lordship at the centre of the town in a quite literal sense. The crannog is believed to be the 'caislean', the castle or fortified seat, of Mac Mahon referenced in Irish annals as far back as 1492. For a period, a map produced by Richard Bartlett around 1602 to 1603 was thought to depict this site, but subsequent research established that Bartlett's image actually shows Roosky Lake, a separate body of water with its own pair of crannogs. The 1591 map, which also shows the Franciscan abbey nearby, remains the clearest historical evidence linking this particular island to the Mac Mahon stronghold.

The mound shelves into deep water within three to four metres of its edge, so the full extent of what lies beneath the surface is difficult to assess from the shore. The four oak pile-tops visible on the northern side offer the most tangible indication of the original construction, protruding at low water and hinting at a more substantial platform beneath the silt and stone.