Hi Rob,OLGA (Online Guitar Archives) is usually the first place to look for chords although they probably don't have less well known folk songs. The other problem is that the copyright police keep shutting it down but it's worth checking every now & then to see if it is back online again.
Here is a page of links to guitar resources online http://www.museweb.com/ag/music.html
The OLGA site address is on there too.
I am a self taught musician too (Celtic harp, a bit of flute). After I started teaching myself the harp I was going to Irish sessions and there was a guitarist/singer who was known as a 3 chord magician, i.e. he could play almost any song ever written with 3 chords, with maybe a minor chord or 7th chord thrown in sometimes if necessary. I learned about the most likely chords for most songs.
By playing the harp it became clear to me what the basic chord patterns are all about. I'll write it offline and post it here in the next day or so.
In the meantime there are probably lots of 'Catters who could explain it to you, and who are much better musicians than I am. We'll see who pops up in this thread.
Helen