At fifteen metres long and just one and a half metres wide, it is a modest enough construction, oriented roughly north-west to south-east, and it rests directly on the lake bed rather than rising above it in any grand fashion. What makes it quietly puzzling is the island it leads to: there are no habitation features on Mearing Island, no traces of a dwelling, enclosure, or any structure that might explain why someone went to the trouble of building a stone crossing to reach it.

The causeway appears in a 1994 archaeological survey of the Ballinrobe district compiled by D. Lavelle, which covers the broader landscape around Lough Carra and Lough Mask. Even then, the surveyors hedged their assessment, noting that the causeway may be of recent construction rather than something ancient. That uncertainty is part of what makes it interesting. Lough Carra, a shallow limestone lake in County Mayo, has a long and layered human past, and the region around it has yielded evidence of settlement going back thousands of years. Whether this particular causeway belongs to that long story or is simply a practical piece of local engineering from the last century or two, nobody has firmly established.