Known as Bolin Island, it is a crannóg, an artificial or artificially modified island built by human hands, in this case by dropping boulders into the water in a circle and filling the central hollow with layers of brushwood and earth. The method was recorded by O'Kelly in 1944, who noted the structure's overall diameter as 90 feet, roughly 27 metres, and observed that mid-19th-century drainage works had lowered the lake level by one and a half metres, connecting the southern side of the island to the mainland. What was once surrounded by water is now, depending on the season, barely separated from the shore at all.

Lough Gur has been a focus of human activity since the Neolithic period, and Bolin Island sits inside one of the most archaeologically dense landscapes in Ireland. Within a few hundred metres lie a Middle Bronze Age house site, a Food Vessel burial, the Early Medieval enclosure complex known as the Spectacles at Drumlaegh, and a dovecote. The lake itself has yielded remarkable objects over the years, including the Lough Gur shield and a gold-decorated spear head, both identified by the antiquarian Wood-Martin in 1886. A crannóg at Lough Gur is mentioned in the medieval text Cogadh Gaedhel re Gallaibh, a chronicle of the Viking wars, though scholars have sometimes struggled to determine whether references in the older literature apply specifically to Bolin Island or to one of the lake's other island features, Garret Island or Crock Island. The 1840 Ordnance Survey six-inch map annotates it as Boilin Island, the spelling varying slightly across sources.

The island appears on aerial orthoimages as a roughly circular form, approximately 35 metres north to south and 34 metres east to west, now covered in trees and scrub. In winter, when the lake level rises, the channel of water that separates Bolin Island from the shore becomes more pronounced and the site reads more clearly as an island; in summer the marshy connection to the south makes it feel continuous with the land. The Knockadoon Peninsula to the south-west offers good vantage points across the lake, and the broader Lough Gur landscape is managed as a heritage site with walking routes and interpretive material that place the crannóg in its wider prehistoric and early medieval context.