I think folk harmonies tend to be quite personal things. When a small harmony group gets together to to a 3 or 4 part arrangement, it tends to develop on the fly. The final outcome is then practiced, but I think rarely written down because of the time and effort required - and of course there is never the intention to publish for "the market". You can get SATB & TTBB arrangements for some folk songs eg Bushes and Briars, Bhier Me Oh, etc which are usually done for choirs.
See if Coope Boyes & Simpson, or Artizan have published any harmony arrangements. I think the Voice Squad is no more now, am I right?
Probably best to just listen to any recodings you can get hold of.
Brad