Well i can definitely concur about the problem with the Big Mac!

I could be wrong about the spelling issue, but as far as i'm aware (at least in Scotland), many people couldn't write, so the spelling of their name was left to the registrar, who spelt it as they thought it should be spelt! Far less romantic or mysterious than a deep rooted family ancestral tale unfortunately!

As a related side note, I can confirm that Mac vs Mc has no relevance to Scottish vs Irish names.