I think it's established beyond all doubt that the original song was about Pomona pleasure gardens in Manchester. Here is Down To Pomona on a 19th century broadside, and if you read the whole of this thread Lyr Req: Down in Albert Square you will see Ian C's journey from believing it of Cornish origin to discovering evidence that it was not.

Although not a native song, it has been adapted and adopted so that it has now acquired Cornish citizenship by naturalisation, just like Little Liza/Little Eyes which originated even further from Cornwall.