This is Dropkick Murphys' take on a Folk song written by Woody Guthrie. Guthrie was a very prolific writer, and many of his songs were never recorded. The Woody Guthrie archives has lyrics to thousands of his songs that are sometimes dusted off and recorded by modern artists.
It became popular after being featured in the Martin Scorsese Academy Award winning film "The Departed".
I'm a sailor peg
And I've lost my leg
I've climbed up the topsails
I've lost my leg