Malcolm is correct: D.K. Gavan wrote the words, Clifton popularized it, and the tune is a variant of "Cam Ye Ower France"/"Keys to the Cellar". Gavan, Clifton or one of their unspecified crones just came up with an arrangement of an existing tune. The major point of interest is that, while "Rocky Road" is now mostly sung as a slip jig (9/8, like the DT tune), the original arrangement for the song was as a jig (6/8). Timing apart, only one phrase leaps out at me as being notably different.