Poland's borders have changed many times and I don't think there is one single body of Polish folk music, but rather music of diverse backgrounds. re Leadfingers comment, The shanty singers I've heard were from the area around the Balkan sea, the east of Poland. I went to a wonderful folk fest in Kazimierz Dolny. I was in Poland in 1995, but I found a fairly recent link to info about the festival at this webpage http://www.polandforall.com/national-festival-of-folk-groups-and-singers-kazimierz-dolny.html