This is interesting stuff thanks Jon!
I think from memory that it was Kathleen Barker who suggested that this was one of the Irish songs that entered Clifton's repertoire between 1855 and 1861 when most of his time was spent in Ireland. He wrote most of his own songs even then, but she suggests that he gained a small number of songs from Harry Sydney and DK Gavan. So it is possible that Sydney wrote it, but I think that the traffic was usually in the other direction and Sydney borrowed a whole lot more from Clifton. (I have started an attempt to systematically go through Sydney's repertoire but it's on hold at the moment, though I do have a scrappy Excel sheet of his songs -its attached in case it's useful.)

I have updated my page to include new information -
http://folksongandmusichall.com/index.php/rocky-road-to-dublin/
and you'll see that I have long been suspicious as to whether GK Gavan is a real person – anyone have any information/thoughts?