What was once described as a very large temple has been reduced, over time, to a faint circular shadow in a field of pasture, detectable today mainly from the air, where a cropmark traces the ghost of something enormous beneath the soil.

Samuel Lewis, writing in his Topographical Dictionary of Ireland in 1837, noted the presence of a very large temple erected on the highest point of Greenish Island. It is a tantalising phrase, and frustratingly brief. What he saw, or recorded from local knowledge, appears on the Ordnance Survey six-inch map of 1840 as a substantial circular enclosure measuring roughly 115 metres in diameter, which places it well beyond the scale of a typical ringfort. A ringfort, for context, is a circular earthwork enclosure common across early medieval Ireland, generally used as a farmstead or settlement, and usually measuring somewhere between 20 and 50 metres across. At 115 metres, whatever stood on Greenish Island was something considerably more significant. The monument has since been levelled, but aerial survey imagery taken in 2005, and orthophotos from 2011 to 2013, reveal a cropmark of circular form with what appears to be a fosse, that is, a surrounding ditch, visible along the western arc. The site sits on the southern side of the island, in low-lying pasture near the confluence of two rivers.

Greenish Island occupies a position where the Deel feeds into the Shannon, a confluence that would have made it strategically and symbolically significant in earlier centuries. Access today is not straightforward, given the island's location in the Shannon estuary landscape, and the monument itself is on private agricultural land. Visitors with an interest in aerial archaeology may find the Google Earth imagery illuminating as a starting point. The cropmark is most legible in dry summer conditions, when differential soil moisture above buried features draws a distinction in the vegetation above. On the ground, the levelled nature of the enclosure means there is little to see without knowing precisely what to look for and where to stand.