Complex sentence: Sean-nós dancing is done exclusively with a solidly-built shoes so that the rhythmic sound of the shoe contacting the floor has a musical quality unto itself, so it can be danced without musical accompaniment.
Split into simpler sentences: Sean-nós dancing is done exclusively with solidly-built shoes. The sound of a Sean-nós dancer has a rhythmic quality unto itself, so it can be danced without musical accompaniment.