the reason why both these singers clubs in Ireland,have a no instrument policy is this,there are many traditional irish music sessions,where singers dont get much of a look in,but plenty of places where irish musicians can play irish music.
so im fairly sure its an attempt to keep it a singing venue,both of the clubs, the spailpin fanach and the skibbereen singers club are well attended,and had a great variety of singers and styles[even though it was all unaccompanied].
I believe when Sara Grey did the Spailpin club she was allowed to use her banjo.Ipreferred to accept the challenge.http://www.dickmiles.com