Well, I haven't played in an earlier addition, but I do know that the original Eldar from RT were Pirates. Also, the Eldar Fleet are made up of the Eldar Corsairs. The Fleets are led by Pirate Princes, who are ussualy assosiated with a paticular Craftworld that they ussauly stay near.
This is about the extent of my knowledge on the subject, though I believe that The Mighty Pikachu and Rasmus were working on an Pirate Codex a while back. You could try talking to them about this.