I've played one, though quite a long time ago. Like Sorcha, I want one of my very own. The extra strings aren't drones, but sympathetics, as has already been mentioned; they ring with what you're playing (you don't touch them with the bow), but typically need to be tuned to suit the key.
Really they belong only to the Handanger area; their adoption as a "national" instrument is relatively recent. There are past discussions here on the subject, which you should be able to find via the search engine ("lyrics and knowledge search" at the top of every page).
More info from (for example) the Hardanger Fiddle Association of America: