Found this which I hope clarifies things:-
"The writer of the song is broadcaster/singer/songwriter Frank Hennessy from Cardiff. As you would expect, Frank is from a Cardiff Irish family, and I believe he gave his permission for the lyrics of 'The Old Carmarthen Oak' to be changed. There really was an ancient oak tree in Carmarthen town, called Merlin's Oak; when it was removed in the 80s the worst floods for over a century followed."
My apologies therefore to Daniel. As you say a lot of the lyric sites provide misleading info.