Re single/double jigs: I'm no expert, but I get the impression that the terms "single jig" and "double jig" are more important to dancers than to players. A "single jig" goes mostly "DUM-dee DUM-dee", while a "double jig" goes mostly "dum-ble-dee dum-ble-dee" (to be pretty simple-minded about it). They're both in 6/8 time. Dance teachers use "Pop Goes the Weasle" to introduce young Irish dancers to the single jig -- that should be a graphic illustration.