This is a bullaun stone, a type of ancient carved rock found across Ireland, in which deliberately ground circular depressions were made, possibly for grinding, ritual use, or both. Scholars have debated their purpose for generations, and no single explanation has settled the matter. What makes this particular example quietly arresting is its position: close enough to the sea that the tide would have been a constant presence for whoever used it, and close enough to a vanished community that it was clearly part of daily or ceremonial life on the island.

The stone itself is subrectangular, measuring roughly 1.05 metres by 0.85 metres, and carries a natural quartz vein running through it, a feature that may or may not have factored into its selection, though quartz held significance in prehistoric and early Christian Ireland and is frequently found at burial and ritual sites. The three hollows on its upper surface vary noticeably in diameter, from around 0.34 metres down to 0.16 metres, and in depth, suggesting either different functions or different periods of use. The deserted settlement nearby gives no easy date, and without excavation the stone's age remains open. Horse Island itself, like many of the small islands scattered along the Mizen and Sheep's Head peninsulas, was inhabited into the modern period before gradually losing its permanent population.