At www.barbarygrant.com, "My Lagan Love" is credited to Joseph McCahill. I found the same name in John Loesberg,ed "Folksongs and Ballads Popular in Ireland" vol 2: "Words by Joseph Mc Cahill, with an ancient air. An American version is called "the quiet joys of brotherhood".

It seems that Joseph Campbell's name has been translated back from the Irish Seosamh Mac Cathmhaoil; I can see how one might come up with 'Mac Cahill'.

I've heard Pete Seeger sing 'The Quiet Joys of Brotherhood'; did he write the words? Loesberg actually does quite well with the notes of his books, as far as I can tell; many songbooks give no background information at all. But calling 'The Quiet Joys of Brotherhood' an American version of 'My Lagan Love' is like calling 'Come By the Hills' a Scottish version of 'Buachaill ón Eirne' or 'Masters of War' an American version of 'The Patriot Game'.