The violin parts feature folk melodies, I don't know what national origin the typical Hollywood piratey/seafaring music from the 18th century was from, but it sounds exactly as you would imagine pirate music to sound like. It's played on an acoustic violin by session musicians only, and they do not have a live violinist.
Official Site: alestorm.net/
Recommended Songs: "Keelhauled," "Back Through Time"
"Back Through Time":
Picture from: