The whole structure connects Green Island to the marshy ground to the north, beginning in stone and continuing as timber driven into the soft tidal sediment, the kind of waterlogged, low-oxygen environment that can preserve organic material for centuries.

The causeway came to wider attention after being identified and reported by Jean-Charles Caillère, who spotted the double post alignment in satellite imagery dating from June 2018. Estuarine causeways of this type, stone-founded routes that transition into timber trackways across boggy or tidal ground, were used in Ireland from prehistory through the medieval period to connect islands, fishing grounds, and areas of seasonal pasture to firmer land. The Owenygarney River drains into the Shannon Estuary, a corridor of movement and settlement for millennia, and the clays that obscure this structure are the same deposits that have preserved other archaeological features across the region. The precise date of construction is not yet established, and it is that unresolved quality that makes the causeway quietly compelling: a line of posts in the mud, leading somewhere, for reasons not yet fully understood.