The main causeway across Ross estuary was part of the new Post Office road between Cork and Skibbereen. It was built between about 1810 and 1814.  As parliamentary papers from London show, the route across the tidal sand flat represented a considerable deviation from the original, planned route, and additional costs were incurred. In the end, the new section of road at Rosscarbery became part of a long-running dispute between George Kingston, supervisor, and William Larkin, surveyor. Nevertheless, just over two hundred years later, the structure remains in good condition and serves as evidence of its good engineering design and strong foundation (Anthony Beese 2017).

‘Twas quite admired by everybody in the neighbourhood.’ (Ann Plumptre 1817).

With the causeway built, this effected the flow of water from Rosscarbery bay and the way the bay flushed out. This caused a build up of sediment and along with the baking heat of the summer sun, an unbearable stench arose. Cork County Council decided to do something about the stench and in the 1970’s they dammed the bridge on the western extremity of the causeway to permantly flood the in-land side of the bay. This would prevent the water level from dropping below 3.2m of the lowest tide at sea level at gives the Lagoon an average depth of 3 feet today.