The song is also known in Italian, and one of the Alan Lomax Italian collection CDs has that version. In Italy, it was part of a ritual involving an actual tree, though I don't remember the details and am separated from my CD collection for the moment (durn job!)

The French Canadian versions are not copied from some Irish or Scottish singer, but likely derived from the French versions. They generally begin:

Derriere chez nous,
voulez-vous savoir mesdames,
derriere chez nous,
voulez-voulez savoir ce qu'il y a?

Il ya-t-un arbre
un arbre d'amour mesdames,
Il ya-t-un arbre
un arbre d'amour joli

It then goes on to talk about branches, leaves, etc.

So there's no bog, and the tree is located "behind our house." The guys I remember singing it in the early 90s were called Hommage aux aines, but I don't know if they're still doing it.

The widespread distribution would suggest it's fairly old, but Neanderthal may be going back a year or two too far!

Maybe, like bagpipes, we can argue about whether the Romans spread it to Britain, or whether it was dreamed up with innate Celtic genius.

Or maybe not.