At least five rows of wooden posts run down to the water in a north-west to south-east orientation, the whole structure spanning roughly 80 to 100 metres in length and 35 metres in width. These are not the remnants of a jetty or a fence line but the skeleton of a working trap, engineered to intercept fish on the tidal flow.

A fish weir of this type worked by funnelling fish, typically salmon or eel, into an enclosure or basket as the tide ebbed, leaving them stranded and easy to collect. The construction recorded at Portdrine shows considerable sophistication. The strongest posts were positioned on the downstream side, where water pressure and the weight of any catch would have been greatest, while heavy branches and poles were woven together as wattle to form the walls of the fence lines. Stones were laid beside the fences to anchor them, and diagonal braces ran to both the east and west, giving the whole arrangement structural rigidity against the current. The site was documented in March 1999 and described in detail by Aidan O'Sullivan, whose 2001 study of intertidal archaeological remains along the Shannon provides the primary account of the structure.