Ok here's my strawman - tear it down if you will.

When a fiddle player leans over to the guitarist and says "This tune is Modal in G" this to me is a sign that instead of a normal pattern that may have the chords G, C , D7 and maybe others, that the guitarist will typically alternate chords between G and F plus a few others. While it may not occur with EVERY instance during the tune the F# notes will be flattened to F most of the time.

The type of modality is very easy to pick up once you know what you're listening to. One of the fun things to do with this form of modality is to mofify existing tunes to give them new life. For example Rakes of Mellow is a great tune that players of all levels of competence can play quite nicely. After playing it a couple of times through in G Major you can kick some new life into it by changing most of the F# notes to F (make it modal), then revert to G Major for the finale. There's lots more I could add but I'll try and post a Midi example in the nexr 12 hours.

You can also take a familiar tune like Soldier's Joy (Irish Version) make no changes to the melody line, but get the guitarist and bass players to substiute F chords for (some) D7 chords. I work this out later. Regards, John