...the instrument does not sound of itself but only amplifies and strengthens the sound produced by the strings, lovely or unlovely... „
The most distinctive feature of the Gaelic harp, both today and to medieval writers, is that its strings are made of metal.
Modern "Celtic" and orchestral harps use strings made of gut, nylon or carbon fibre, and historically the usual string material for European musical instruments including harps, was gut. However from the earliest times the Irish (and the Gaels in Scotland) are said to have used strings of metal on their instruments.
So for musicians today who wish to explore historical Irish and Scottish harp music, stringing practice can be the most contentious issue: are brass, steel or phosphor-bronze strings more "authentic"? Should bass strings be plain, overwound, twisted, or even silver or gold? Does it actually matter?
I hope Bendeler's quote shows it does matter, though the other questions can be less easy to answer...