The Morris Federation's website shows this tune is used by Shinfield Shambles Border Morris for a dance called Castle of Mey. There's no information on the tune itself. It appears to be a dance of their own devising, so perhaps the tune is too.

https://morrisdancedatabase.org.uk/tunes.php?action=tune_search&name=mermaid

Shinfield Shambles'# Facebook page says they're folding so if you're going to try to contact them make it quick

ABC tune finder the Traditional Tune Archive both have several tunes of that name.