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.