What about Church singing clubs? and what about Trade Union 'doos' both of which were long established before WW2. Also here in the USA 'pickin' under a shade tree on a lot of town squares esp at the weekend when folks would be in town getting provisions for the week, not like the Music Hall atmosphere of the Folk Club, but nonetheless much the same thing in a lot of ways.

I suspect like traditional Irish house dancing or it's American equivalent, there isn't an exact starting date, more likely a gradual drifting towards a new way of doing the same old things. Compare for example todays 'sessions' and the now almost extinct 'kitchen celidhs'.