Found out about this beach by searching for beaches on Ibiza online. It's SE of Portinatx and at the end of a road that at first goes past the entrance to a couple of villas before degrading into an unsurfaced track and winding it's way down to the un-finished shell of a large hotel.
It's a nice little beach in a nice little cove which has seen very little development. There is a small beach hut that does drinks and snacks and has a few sunbeds for rent and a hotel that no-one has cared to finish building which has become overgrown with plants.
There were probably only 20 other people there when we went down on a sunday including a few locals. The sand was a little pebbly but not too bad and after a couple of meters in the water the surface becomes rocky underfoot.