In my opinion Folk music is quite a broad church, a a performer I tend to sing mainly trad material, but that does not mean that I do not appreciate other forms of folk music apart from those I sing.
it is quite obvious from looking at guest lists of folk clubs what kind of folk music you are going to get, as regards singers nights it is very easy to look at the clubs website and get an idea from videos of floorsingers or residents what you might be in for.