Allan - obviously there are factual questions which can only be answered one way: MacColl was thinking of Salford when he wrote Dirty Old Town, and there's no such song as "Shoals of Erin". When you get into what a song means, though, I don't think the writer can have the last word. The reason why people can find a racist message in "Roots" - or a pagan message in "Lord of the dance" - is that the words can be read in that way, irrespective of how the author wanted them to be read. (I suspect Sydney Carter wouldn't have minded the pagan reading of LOTD, though.) I found some interesting critical commentary on "Roots" here and here.