It seems that Jack, you have nailed the history of the word. I noticed an interesting progression in the Irish collections:

Bunting (with collections dated 1796 and 1840) has no tunes named "Humours of..."

Petrie (collections dated 1855 and 1882) has one tune starting with the word.

O'Neill (1903) has 31 tunes named that way.

From these dates and the dates of the Scottish tunes listed by Jack, it would seem to have been first a Scottish naming convention, originally in English, which then migrated to Ireland.

Cheers,

David