Interesting discussion, this. Murray's comment about current scottish accompaniment, as typified by Hazel Wrigley, is a good one.
However, my own preference as guitar accompanist to Scot/Irish tunes that have a strong rythmn, is generally for a more percussive sound, a la flatpick.
For ballads, and pieces with a looser rythmn, i like to fingerpick/arpeggiate some of these--depending on what sounds best...ymmv, though. I tend to fingerpick mostly with a flatpick and two fingers now.
And much of my guitar accompaniment is strongly influenced by my other primary instrument, upright bass. Anyone else find that their guitar-ing is influenced by the other instruments you play?
My choice of accompaniment style will vary too, depending on the timbre and range of the instrument doing the lead.