Is it restricted to Irish tune names?
This use of the term might ultimately derive from humoral theory, but if so then it's odd for it to be restricted to Irish tunes; that makes me think an explanation closer to hand would be more convincing. Could it not rather be a by-word for 'air' or a translation of an Irish term? Jack's suggestion above (humours of Glen as a sort of paradigmatic example that gave the term currency) also seems an attractive suggestion.