Traditional Irish Folk Music 13 Song Styles 4 Scales

Scales

As a generalization the majority of Irish folk Music is played in the keys of D Major  and G Major. The Session Website(1)has quite a long discussion as to why this is yet no definite answer appears.

 The scales or modes most common in Irish Folk Music are

Ionian or Major

Dorian ( a minor mode)

Mixolydian  (a major mode)

Aeolian or natural minor 

Taking the two keys into account we get the following

Scales based on Dmajor:
-----------------------
D major/D Ionian:  D  E  F#  G  A  B  C#  d
E Dorian:                     E  F#  G  A  B  C#  d  e
A Mixolydian:                              A  B  C#  d  e  f#  g  a
B Natural minor/Aeolian:                B  C#  d  e  f#  g  a  b

Scales based on Gmajor:
-----------------------
G major/G Ionian:  G  A  B  C  D  E  F#  g
A Dorian:                    A  B  C  D  E  F#  g  a
D Mixolydian:                            D  E  F#  g  a  b  c  d
E Natural minor/Aeolian:              E  F#  g  a  b  c  d  e

So we have two keys with predominately the same notes. Perhaps the reason those keys are    the most common, is simply that they have the easiest notes to play on the instrument or the      easiest fingering.

Quite often a 'gapped' scale is used where the third or sixth is omitted, the seventh is also          sometimes subject to this omission. Quite often this makes the tune sound archaic.

The third and Sixth  are also occasionally 'half sharpened' raised a quarter tone in pitch. 

Reference:

'Why are most irish tunes in D or G ?' (forum)[online] available at:<http://thesession.org/discussions/25437> (accessed 5 October 2013)

Speek Han, '"Chord Scales" and accompanying Irish dance music.' 1996 (article) [online] available at: <http://hspeek.home.xs4all.nl/dadgad/theory.html> (accessed 5 October 2013)