Captain Birsdeye makes a very common(and understandable ) mistake when he says that Irish polkas are modern tunes. Not necessarily. The polka, like the waltz, is a dance that didn't hit Ireland till the 1800's. But, as is often the case, old tunes became used for the new dances. Many, (perhaps most?), of the tunes called "Irish polkas" were not written as polkas, but merely pressed into service for practical use.In fact the generic term "Irish polka" is used to apply to tunes that mostly aren't polkas in the normal sense of the word. The same with waltzes: many of the old country dance bands, in Ireland and Britain, played medleys of waltzes, which contained loads of tunes which long predate the arrival of the dance called the waltz.
    I stand by my theory: statistically, minor tunes gradually went out of fashion in the period 1600 to 1800, in Ireland and in Britain. They didn't vanish, obviously, and people were writing new ones all that time. But they always being out-numbered by even more new major tunes. This continues to the present day in pop music(listen to any chart song radio programme); but in trad circles the minor tunes are making a bit of a comeback as they provide a welcome contrast in tune-sets at sessions.