This map from 1868ish shows a small island to the south of the harbour, pretty much in the area we knew as Monkey Island when we were kids, but it might be the quarry, it's difficult to tell. I've rather handily put a red ring around it. (Just for the record I wasn't around in 1868). It would be nice to overlay this old map onto an up to date one to see all the changes. I'll have a go at that over the winter.
https://db.tt/3ZTM6Avi