Thinking now, I'm not sure I'm consistent on the classical one. I'm not always fond of classical voices for "folk" but John McCormack works for me as does say Bonnie Mary of Argyle on this

A mostly Irish session I sometimes get to has part of Handel's Royal Firewoks as an occasional tune. Dit dit dit dit deeddle deelele dee.

I tend to have a harder time with some varieties of singer song writers. But that's just me. I might even prefer a more rigid definition of a "folk song" but I do consider what may work in a given say "folk club" setting a different matter. The venue may or may not in varying degrees be for me but I can find things that suit me in the variety we have.