Ewan MacColl certainly wrote it but only in snatches for different radio shows. I saw Louis Killen many years ago and he claimed to have put it together as a song. He even laughed about Ewan being asked to sing it and not being able to.

The version on the Database is different from the Dubliners (Luke Kelly) version I am familiar with. I'm sure there was a verse which mentions "scraps of Navigation". Also the final verse is missing. It's sung to the same slower air as the first verse and goes something like: "Night and Day the seas we're faring
                Come winter winds or winter gales
                Sweating or cold - gowing up, growing old or dying
                As we hunt the Bonny Shoals of Herring"

Terry