See no reason not to take the OED definition: "A harp tune of a sportive nd animated character, moving in triplets. It is not intended for or often adaptable to words and is slower in pace than the jig." (Stainer and Barrett). Derivation unknown, app. not native Irish (Petrie, Ancient Music of Ireland, 1855). In print in 1790. Some derive from the Latin plangere, to strike or to beat.