Well done Dave, that's exactly where it is
I don't know the exact reason why it was built but I believe that it was as you said built for access for some type of workings
My guess is that it was for the quarry behind the opening. When I had a look at it you could see parts of a metal ladder that went from the shore up to the tunnel. There was also a wooden pier at one time that enabled boats to transport the material by sea and a few wooden stumps are still visible there