People often ask me questions like these. I wonder how I can squeeze years college music theory & decades of playing & listening into one brief paragraph which will explain everything they need to know. I have not yet figured that trick out. I do, however, remind others that good ear training may be found in lessons with a good instructor, and also in many of the guitar method books--choose those which have been around for many years. One must also not overlook the other obvious things such as local jam sessions where we often find experienced players, and, also listening to many of the recordings of old time fiddlers. Irish/Celtic music is often not the same chord structure as Bluegrass, and when we play "Celtic" tunes, we must remember that many of them have been passed down from the early musicians who played the tunes on bagpipes. The pipes are not fully chromatic, and the tunes often sound modal. Tunes such as "Campbell's Farewell to Redgap" have only 2 chords. Yes, 2!! If the tune is played in the key of A (since most folks can't play in Bb, they play in the nearest "Bluegrass" key, which would be A), then the chords are A and G. This is also the old timers way of playing Old Joe Clark. By using this "modal" sounding chord pattern, the fiddler is allowed to have more notes in the scale to choose from! Tunes have been altered over time, but if we listen to the tune carefully, bearing in mind the basic chords in that key, we usually have a fighting chance of hitting them. Fiddlers often wander all around the melody, and we must not mistake some of that wandering as needing chord changes. I came up playing in Ceoli bands, and most often, the fewer chords, the better. That is the traditional way. The old timers did not frequently make use of sevenths, or fancy stuff. they played very basic chord patterns. If traditional music is to be passed on, it's up to us do do it. It's wonderful that you have such an interest in playing with fiddlers, as that is how it was often played in the "olden" days.