The Gaelic song tradition in Scotland is very diatonic.  A strong relationship would
have existed between song and the diatonically tuned harp or lyre.  There is a higher
level of chromaticism present in the Gaelic song tradition in Ireland which sometimes
makes it difficult to decide where to situate some Irish songs on the diatonic gamut
of the Gaelic harp.

The harper could not change tuning manually while playing.  There is evidence for the
existence of fully chromatic harps in the Irish tradition and even more evidence for an
influence on Irish song from chromatic English song.  However, the harp tunings
provided by Edward Bunting cater for a mostly diatonic harp tradition and some
chromaticism only at the octave, eg the F# being sounded only an octave below F
natural and not a semitone above it.  The Gaelic harp/song tradition in Ireland
therefore seems to have been strongly diatonic as in Scotland.

A Gaelic harper of old would have known where to situate particular tunes and their
basses on his wide-ranging instrument.  Any harper working with a singer, as per the
historical model of harper and reciter, would need to know how to set his instrument
so that certain types of tunes, with certain scales and certain melodic ranges, could
be fitted on the harp gamut in a position appropriate to the singer's vocal range.  
These articles are an enquiry into the musical system of the Gaelic harp.
Gaelic modes
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by Alasdair Codona