I've been involved in singing workshops specifically related to folksongs for over forty years; I have long held the opinion that, while folk singing can be learned, it is extremely doubtful that it can be taught; not by individuals at least.
You can learn a certain amount by listening to others and attempting to imitate what they do (for practice only) in order to get used to using and exploring your voice, but this, I believe, is quite slow and can be hit and miss (along with running the risk of becoming a clone).
The most successful set-up I experienced was the group situation with a number of people, singers and non-singers, of mixed abilities and experience all rowing in to assist each other. This can work on various levels - as unplanned get-togethers of enthusiasts casually meeting to discuss each others singing with a view to solving specific problems, or the more organised regular sessions aimed at facilitating the abilities and understanding of the whole group. The former can, in my experience, be of some help, but the latter can have of long term value and can produce some very fine, long term work.
The more organised set-up needs to be planned and structured with at least one experienced singer willing to devote their time to the venture.
I believe that any folk club that is serious about promoting the music and developing new singers should think seriously about workshops as part of their work - in my experience there have always been plenty of people around more than willing to help.
I don't believe that general 'singing teachers' can be of much help unless you can find one who is specifically knowledgeable in folksong styles; I've yet to meet a classicist who hasn't described the open 'folk' voice as "ugly", and other types of song require other types of voice.
Here in Ireland an upturn in the in the interest in traditionional music has led to singing classes and teachers springing up everywhere like mushrooms. The few I have experienced have left me with the impression that the technique used is, at best, helpful only in introducing would-be singers to songs, but does little in encouraging them to find their own voice - essential, to my mind.
A controversial point - but one I'm absolutely unmoveable on - DON'T PRACTISE IN PUBLIC, don't stand in front of an audience before you have at least learned to hold a tune and are able to remember and become comfortable enough with the words to understand them and are able to communicate them to the listener. It's not fair on the audience or on the regular singers in the venue, and from your own point of view, should it go wrong and the song fall apart, it's just like the Sisyphus Beetle's dung-ball rolling right down to the bottom of the hill so it has to start again. There are other ways of getting experience of singing in front of people.
Singing is like every other artistic endevour (that's what it is) - the more you work at it the better you become.
It's a great feeling to stand in front of an audience and not make a hames of the song, hold the tune, remember all the words without having to read them of a ****** crib sheet.
But to make a GOOD job of a song, to know you've made a good job, and to know that the audience have enjoyed your song because you made a good job - that's the flag on the top of Everest.
Jim Carroll