As I read Frank Hamilton's reply, which I essentially agree with (except that we have some disagreements about some of the subgroups) I am reminded that the question of what constitutes folk music and what doesn't is not really an open question--
Folklorists and ethno-musicologists, as well as various sorts of anthropologists and sociologists spend their lives collecting analyzing, and categorizing this stuff, and there are all kinds of useful distinctions that have been made, all for the purpose of resolving the sort of questions that people bring up here--it just seems that people don't want to sit still for the explanation--
Handel's Messiah is not in anyway folk music--it is composed music--the piece may include melodies that like those that the village sings, but the melodies are only components in a larger structure, which in itself is only a vehicle for the expression and development of Handel's unique musical ideas--not folkloric at all--
There is a bigger issue here, though, and that is that, if we are compelled to classify Handel's Messiah as folk music, when it is such a clear embodiment of classical music, it destroys the the definitions that have been created to analyze musical and cultural phenomena--and without ability to define, the discipline, and all of it's work is destroyed
This makes it impossible do any type of analysis of either music or culture, and in the long run, it makes it impossible to create music, and it probably undermines the process of maintaining other types of culture as well--
I don't mean to pick on McGrath, whose real point I have probably totally distorted to make my own (sorry!!), but it was the only example I had to work with--
I get more and more disturbed every time a question related to this subject comes up, not because I am tired of the questions, but because it seems like the questions are used as a way of attacking and undermining the process of finding answers..