This concrete structure on Marloes sands is sheltering a rare plant - Rumex rupestris or Shore Dock. But clearly this structure is a cable landing. Donard de Gogan thinks he knows what it must be a relic of:
"Haverford West was one of the Post Office relay stations en-route for Ireland. Indeed, W.T. Scaife worked there before coming to Valentia. He was Superintendent at VA during the first world war. Cables to Ireland ran into Northern Ireland, Howth (near Dublin), Newcastle (Co. Wicklow) and Wexford. I suspect that you have found the cable hut for what were the lines from London to the stations in south west Ireland. Various company correspondences indicate that they were never quite sufficient for the demands put on them and the CCC cable into Weston was of course the first attempt by a private company to circumvent this bottleneck."