I found another intersting quote from Jim Carroll, re Ewan and songwriting
" had been staying with them one time while Ewan was making tunes for his songs; he would pick a source tune and wander around whistling and humming it under his breath, adapting it as he went until he reached a satisfactory conclusion - drove everybody barmy.
What I got from these experiences never left me - I now find myself trying to match up tunes with others to see if they are related      
I came to the opinion that Ewan's favourite 'adaptable' tune was the air of 'Famous Flower of Serving Men' from Gavin Greig's 'Last Leaves of Traditional Ballads'.
https://soundcloud.com/ewanmaccoll/sweet-william-the-famous
This was used for 'Shoals of Herring', 'Freeborn Man' and at least four more
https://www.youtube.com/watch?v=6Ov81aogaxg
'Tunnel Tigers' was adapted from a version of 'William Taylor' sung by Paul Lenihan, an Irish singer living in London."