Hi, Peter.

In notes to 'OTM Transcription: Spanish Fandango', Tony Russell commented [cf Old Time Music#6 Autumn 1972, p12]:

Old time musicians know many guitar showpieces, but few are as widespread as 'Spanish Fandango'. Few, what's more, go back so far. The origin of 'Spanish Fandango' is uncertain, but it is clearly of the 19th century rather than the 20th century, belonging to the parlour tradition from which came, say, 'The Siege of Sebastopol'.

'The Siege of Sebastopol' was published in 1880, and it probably explains why rural musicians call open D tuning 'Sevastopol'. Similarly, 'Spanish Fandango' may provide the origin of 'Spanish' as a rural musician's regular term for open G tuning. 'Fandango' is commonly performed in open G.

Such theorising explains why the tune is so widely known - as theories about an actual Spanish origin do not.

I hope the above is of some use to you, Stewie.