Another good resource I've used in a book/CD set called "Celtic Back-Up for All Instrumentalists" by Chris Smith. It includes charts of common chord progressions for the most common folk music modes in various keys, and he has a comprehensive set of tools (bass drone, bass runs, countermelodies, etc.) to vary backup, and all sorts of good suggestions. I think Smith has his own website (don't know the address). The set is available at elderly.com and elsewhere.