If you are doing web searches, it will help to spell "Dungarvan" with an "a" at the end. Also, apparently there is a recording of the song on which the label mistakenly reads "Old Dungannon Oak", and the song is derived from the "Carmarthen Oak"

See this thread: Old Dungannon Oak
"... I might be able to help on the story of The Dungarvan Oak v. The Carmarthen Oak. One of Frank Hennessy's early songwriting success'was a recording of his song The Gypsy by an Irish singer called I think, Dermot Henry. Another Hennessy song was requested for his follow up. At the time he hadn't a suitable Irish flavoured song, but he did have The Carmarthen Oak, which tells of a local legend. So,the title was changed to Dungarvan Oak, because it fitted and Hennessy had conections with the area, a couple of words of the chorus were changed, and hey presto, another big Irish hit!. It has since been recorded by The Bards, that too was a hit.The strange thing is that some people now believe that there is a legend about an Oak tree in Dungarvan. Out of this legends are born!