Not a koto - I think michaelr's bass hammered dulcimer is a more accurate description of this (home made?) instrument. What it's doing in this context is anyone's guess. I still love the old School of Scottish Studies recordings of Waulking Songs (Music of the Western Isles), but we seem to drifting into the realm of Celtic Woman here - but then again Alan Stivell was popping them up on Journee a la Maison many decades ago, Clannad likewise, so why not?

http://www.youtube.com/watch?v=XD3pDSPsl4M