There are two native Appalachian instruments called "dulcimers", one hammer, one lap. Both seem to derive from instruments brought into Northern Appalachia by the so-called "Pennsylvania Dutch", their provenance being German, not "Scotch-Irish".

It's odd, therefore, to see both instruments being often associated with Irish traditional music in the USA.

I think it was started when Derek Bell, on The Chieftans 5, played in addition to harp, a Romanian folk instrument called the cimbalon. Rather than calling it what it was, the The Chieftans called it "a reconstruction of the ancient Irish tiompan." Tiompan is one of many words for musical instruments that occur in old Irish writings; too bad that we don't know what these words actually refer to. So The Chieftans calling Derek's cimbalon a tiompan was all about marketing and not so much about historical accuracy.

Anyhow The Chieftans go on to do concerts in the USA and everyone sees Derek playing this thing that sort of looks like an Appalachian hammer dulcimer and sounds very much like it, and Americans make the leap that the Appalachian dulcimer is Irish.

Nowadays many if not most hammer dulcimers are made specifically for the USA Irish music scene and many have Celtic decorations on them etc.

Likewise the Appalachian lap dulcimer has made this odd leap and I've seen them covered with Celtic decoration etc... what an odd fate for a German zither!