That they have been everywhere since the earliest times is not in question & a number were found, from the late Medieval period, at a dig in Carrickfergus: 
It is pretty certain too that they were in common use by the Ulster-Scots folk, especially when you consider that in the early days in Appalachia - "The most popular instruments were the jew's harp and fiddle. Less often one would encounter a plucked or hammer dulcimer, some other home made instrument or possibly a flute or fife."
This extract was taken from Mike Seeger's' 'Music of the Southern Appalachian Mountains': 
Of course they are a relatively quiet instrument & while they would have suited perhaps a small friendly kitchen ceilidh, playing along quite comfortably with a Fiddle, in the days before TV, today they simply can't compete with loud Accordions & Bagpipes & are totally unsuited for large dance & concert halls.
However, in a recording situation, they do add a certain magic to the sound an ensemble makes, especially if they are in tune with the instruments & the player is actually skilled enough to play melodies on them!
Listen to a tune being played on a Jews Harp, in duet with a Harmonica - do you recognise the tune: 
For instructions on how to play the Jews Harp, visit the excellent website of 'Michael Wright': 
Here's a short instructional Video Clip by 'Terje Isungset' on how to play a Jews Harp: 
Listen to Jack Elliott playing the Strathspey 'Brochan Tanna' on the Jaws Harp:
Watch & listen to John Fowler playing the jawharp in the Appalachian style: 
To find out much more about the Jews Harp visit the Jew's Harp Guild site: 
Here is an interesting 'session.org' discussion on the Jews Harp: 
There are lots of Jews Harp clips to listen to on this page: 