@Malcolm: "I had quite forgotten that Jack Campin pointed out in another discussion that the tune is basically a 9/8 version of the old 3/2 hornpipe 'Cam Ye Ower Frae France', also known as 'The Keys of the Cellar'."

Cam Ye O'Er Frae France doesn't sound anything like the Rocky Road to Dublin, even in a different time. I've listened to numerous versions of it, as well as Rocky Road...

The Wikipedia page for Rocky Road to Dublin repeats this claim, but the reference it provides for it shows no evidence that it is based on Cam Ye O'er Frae France.

All the reliable references I've read said the source is unknown.