Many Scottish dance and other traditional tunes are written in the MIXOLYDIAN mode so the normal dominant seventh chord in the normal three chord trick would be VERY WRONG. In the key of "G" mixolydian the accompanying chords are generally G C and F (instead of D7), in the key of "D" mixolydian they would be D G and C (instead of A7) and in the key of "A" mixolydian they would be A, D, and C (instead of E7). Why is this so? It is because the seventh note on the scale has been flattened by one semitone, ie in the Key of "G", F natural is played instead of F#; in the Key of "D", C instead of C#; and in the key of "A" G instead of G#. Guitar players who simply play the traditional three chord trick always wreck the tune if they do not recognise this modal change. Further steps into the modal cycle involve flattening other notes on a prescribed basis. Many of these modes are not used regularly but the DORIAN mode turns up now and again. The appropriate accompanying 3 chord trick in the Key of "D" dorian being D, F and C.
It is not rocket science but it is far from common knowledge, indeed, modal music is generally so poorly understood that there was a conversation between Cecil Sharpe and a very senior person at the Royal College of Music that is reputed to have contained the remark "Do you mean that these peasants understand mixolydian and other model music when I spend years trying to teach my students it without much success?" The reply was "Yes.".
Take care though, sometimes Scottish music fluctuates between major ( or called in modal terms Ionian) and mixolydian keys as in 'Athol Highlanders' other times it stays firmly in the mixolydian key.
One way of a guitarist spotting is to have a look at the number of sharps at the start of the tune and the final note. If there are 2 sharps the end note would normally be D but if it is A the tune is probably in A mixolydian (probably being derived from pipe music). One sharp finishing in D probably means D mixolydian, and no sharps finishing in G is probably in G mixolydian. But beware music is an art and not an exact science and although this rule may well be of assistance it is not immutable.
Chief O'Neils favourite fluctuates between the major and the dorian modes and whilst the majority of the tune is in D major the first two bars of the b part are in dorian mode and thus require F and C chords (one bar of each) to obtain a correct accompanyment.
And If the odd guitat player thinks I am being harsh, I am one too, but take the view that I accompany the tune and shoud therefore play appropriate chords and not simply crash on with a standard 3 chord trick.
Most of modal music is a mathematical progression of the 1 or 2 semitone gap between notes, I did produce a piece of work on this but I am not sure I can open or up-load it as it was on a data base that my current set up would not recognise. When I get a decent scanner I could try to get it into a form that might be transportable, in the meantime sorry can only get a copy to someone via SAE and snailmail.