I have a little booklet named 'Blodau'r Grug - 32 Traditional Welsh Airs for Dancing Vol. 1' It has a dated format (clearly pre-computer) and was published by The Welsh Folk Dance Society. No date.

#8 is Ffarwel I'r Marian. I think we can assume that this is an old dance tune, and it's in the public domain.

Me, I don't believe it means Farewell to the Shore. Nobody wrote that beautiful, yearning tune to a cold, wet, gray stretch of water.