Who would have thought to pair up Celtic Woman and David Archuleta? Okay, so this not exactly Celtic Woman. Orla Fallon (she's the one with the flaming red hair) who used to be a member of Celtic Woman had her own Christmas special this year and David Archuleta was one of her guests. This is the same arrangement of Pat-a-Pan that was on David's Christmas album last year. Orla is the backup singer prominently displayed over by the grand piano.
(Last December I posted a fan-made video for David's Pat-a-Pan that is one of the best I've ever seen. You can find it here.)