"The famous example is Ewan McColl's "Shoals of Herring"
Moot point Howard; Shoals of Herring appeared on the Irish scene some time after the Irish song tradition, certainly the English language one, had shuffled off this mortal coil.
The reported hearing of 'Shores of Erin' came from American scholar Horace Beck, reported in his 'Folklore and the Sea' and it was never established whether or not it is based on his hearing (or mishearing) of the song sung by a bunch of folkies who got it from a Dubliner's album. It certainly never underwent too many changes which, I believe qualify it as being traditional; not did it take root to any depth here in Ireland outside the ballad scene (Ireland's folk revival).
There is more of a case to be made for MacColl's 'Freeborn Man' which possibly would have entered the Travellers' tradition, had it survived beyond the mid seventies; but even that only appeared as mangled fragments, again, apparently from misheard Dubliner's renditions.
The groups themselves had a hand in making some, (often ludicrous) changes to the texts of songs - beatiful example with The Johnsons' 'version' of Tunnel Tigers
Jim Carroll