"Pentatonism is no mark of anything. The oldest documented Scottish and Irish music is very firmly heptatonic, and pentatonic modes are found all over the world. If anything pentatonism is often a recent innovation in Scotland and Ireland rather than a relic of anything ancient."

This is true of composed music, but it probably does not apply to folk melodies. Among British folk music, 5 and 6 note 'gapped' scales are much more common in northern Britain than in southern England. This probably represent more a survival of older approaches to melody than innovations borrowed from modern composers. The gapped scales and circular melodies that fascinated Cecil Sharp in the Southern Appalachians certainly cannot be traced to the innovations of modern composers. While I don't believe scales and melodies of this sort are specifically 'Celtic', they have lingered (like bagpipes and drone notes) in regions that have been labled (however accurately or inaccurately) as Celtic.