Though the Oxford English Dictionary has no mention of the Irish "derivative" (for lack of better term)...but I looked up "willow" and "garden" in my Irish/English dictionary (use it for various words in songs only--don't speak the language) and found this:

saileach (willow) nominative case-- sail

gairdín OR garraí (garden)

someone told me "willow gardens" in Irish would be
"gairdíní saileach" [willow garden = gairdín saileach]
and yet someone else said "garraí sailí", though I guess
it depends on how it is used in the sentence....

So it could very well be that Sall(e)y Garden(s) is an anglicised version of the Irish words...

SingsIrish