"Bring us a Barrel" by Keith Marsden. In "Picking Sooty Blackberries" It's explained that when he first started singing it, Keith told people that he'd collected it from a traditional source. It was several years before he admitted that it was his own composition.

Many of the best songwriters produce songs which could easily be traditional. for example:

Dave Webber: "Bonnet & Shawl", "Watch & Chain" + many others
Bill Lowndes: "Old Fid", "The Sun and the Moon"
Stan Rogers: Barretts Privateers