Just because you find a version of a song in a minority language or dialect doesn't automatically make it older than the English version: probaly more songs have been translated from English into Gaelic than the other way round.
Sus man j'valet does indeed appear on a website of Jersey songs, but if you look at the index page of that site you will find many other "traditional" Jersey songs which have subsequently been translated into English such as
Bridge over Troubled Waters
It's a long way to Tipperary
All you need is Love
Auld Lang Syne (so that's where Burns stole it from!)
My Bonny lies over the Ocean
Singin' in the Rain
etc., etc., etc
Perhaps then we might fruitfully look to see if there is any evidence of Sus man j'valet before the year (say) 2000.