That a learned American researcher thinks of "Shoals of Herring" as being typical of the songs sung by Kerry fisherman doesn't surprise me, I mean the words fit so well don't they!

The researcher was presumably talking about the style and the way the song was constructed, not the specific placenames. MacColl's song is like nothing in British tradition, so well done that researcher in finding something that might have inspired it. Why shouldn't it have been a pastiche of an Irish musical idiom? From somebody who thought he was imitating Japanese melody in "Schooldays End", you don't expect British nationalist purity.