The Butterfly is as much about reinvigorating the Irish fiddler tradition as it is about the string quartet reaching beyond its comfort zone. Martin Hayes and Brooklyn Rider explore each other’s worlds in magical ways: The quartet lends classical color to the opening track, “Maghera Mountain,” that serves as a gentle breeze beneath Hayes’ wings. And playful pizzicato bass and double-stopped drones bring Brooklyn Rider to the dance in “Jenny’s Welcome Home to Charlie.” “Bob and Bernie” hesitantly emerges from its echoes of Samuel Barber into the sunlight of a joyful reel. But nothing in the collection is more moving than the gentle string whispers and murmurings that, together with Hayes’ plaintive cries, give “Port Na Bpúcaí” a breathtakingly calm, mystical quality.