Twenty roundwood stakes, spaced roughly twenty centimetres apart and running eight metres in a north-west to south-east line, are all that remain visible of what was once a fish weir, a structure designed to trap fish as tidal waters receded. These kinds of intertidal weirs were once common features of Irish estuaries and coastlines, typically consisting of timber post alignments or stone walls arranged to funnel fish into a confined space where they could be collected at low tide. This one is eroding out of the gently sloping estuarine clays of the Fergus shoreline, exposed by the same tidal forces that will eventually destroy it.

At the north-western end of the alignment, a cluster of roundwood posts sits submerged in shallow water, and researchers have suggested this may have supported some kind of trap, the business end of the whole apparatus where the catch would have been held. The date of the structure is unknown. Fish weirs of this general type have been found across Ireland ranging in age from the early medieval period through to the post-medieval, so the Carrownanelly example could belong to almost any point across a very wide span of centuries. What is documented is its physical form, recorded by Aidan O'Sullivan in his 2001 survey of the Fergus estuary, which catalogued the intertidal archaeology of this stretch of the Shannon's tributary in considerable detail.

The Fergus estuary is a tidal, muddy environment, and what survives here survives only because waterlogged conditions slow the decay of organic material like wood. The same erosion that reveals these posts is also wearing them away, meaning the structure is better understood now as a record in the process of being lost than as a monument in any conventional sense.