Where did that attribution to Rory Dall O'Cathain attribution come from? Bunting, 1796, didn't give that (although he gave the title in English as well as Gaelic), nor has anyone else that I know of. Besides, the tune was given twice in 'The Scots Musical Museum' before Bunting published it. Also "The Young Man's Dream" is an 18th century song based on a 17th century English one.