I do quite a bit of harmony singing, especially on choruses. I can't really explain it any other way than I heard Chris Sugden (aka Sid Kipper) explain it, 'you find another tune that fits'. The folk club I went to in the 60s was started by the Watersons, so harmony singing was the thing to do. I remember guests who hadn't been before being quite taken aback by the strong multi-part harmony singing on choruses, as it wasn't something they were used to. My partner, Maggie, is a good natural harmony singer too.
When we've sung like this in Ireland, with harmonies, it's always been received very well, although it's something they don't hear so often - as Noreen says it's not a tradition over there. The Voice Squad (who anyone interested in harmony groups should listen to) were, I think, more inspired by English harmony groups.
I suppose what we are looking for here is, how long does it take for something to become a tradition.
Thanks for bringing this up, JJ. I'm look forward to other contributions.