Regarding the 'Carmina Gadelica', a quick google shows me that this text is suspected of being mediated. It almost goes without saying, I am coming to think. The wiki entry suggests that it is not generally thought of as presenting a 'literal' account of what was happening in the oral tradition. Regarding who wrote the various pieces contained therein, I do not know what information is available. So I don't feel that this text is particularly useful in arguing that 'folk' stuff was originally written by the lowest class, even if some very very poor people were singing it.