I,ve just seen this thread again and I find it amusing how it has affected some people. I wrote it as I did many other historical songs, based on research and recorded facts.
Jim Lad, Mc and Mac or M' (open quote) are all the same. There is an intersting page in the notes to Johnson's Musical Museum where the name McGibbon is spelled McGibbon, MacGibbon and M (small, underlined c), all, I repeat on the same page. It's purely a matter of spelling in the 18th and 19th century and all would have been pronounced exactly the same.