Tobar Leo, or Toberleo, is a naturally formed rockpool sitting on a ledge, fed by a spring seeping from the rock face above it. Its irregular shape is determined entirely by the stone around it, with no masonry, no structure, and no obvious human intervention. The name follows the Irish convention for such sites, "tobar" meaning a well or spring, though the distinction between a true well and a natural basin fed by groundwater was rarely the point.

The site sits on the eastern side of a sea cave or inlet known as Fuagh Leo, recorded on older Ordnance Survey maps as Ooghleo, on the island's southern shore. References to it appear in the early twentieth century work of Thomas Johnson Westropp, the prolific antiquarian who documented sites across the west of Ireland, and in a 1927 compilation by O'Flanagan. Inishshark itself, sometimes spelled Inis Airc, was permanently evacuated in 1960 when its remaining inhabitants were resettled on the mainland, leaving behind the ruins of houses, a church, and various features like this one that had formed part of everyday and devotional life. Holy wells in Ireland were, and in many places remain, sites of pattern days, local pilgrimage, and cures attributed to the water. Whether Tobar Leo held that kind of active ritual life is not recorded in the early sources, but its naming and documentation suggest it was regarded as more than an ordinary rock formation.