most trad songs and tunes were passed down by word of mouth or ear,and names developed over a period. thus a tune may have many titles,so it is sometimes easier to class them as,say, kerry polkas.
mally publications do a 100 best polkas,and like his other books he lists alternative titles which can be a good way of learning the names. but it is annoying to haer a tune you like and fancy playing,only to be told "oh,it's a kerry polka"