Eoin Mac Neill,
Phases of Irish History
“ By what criterion do we recognise any ancient population to have been Celts? The answer is undoubted—every ancient people that is known to have spoken any Celtic language is said to be a Celtic people. The term Celtic is indicative of language, not of race. We give the name Celts to the Irish and the Britons because we know that the ancient language of each people is a Celtic language.A certain amount of enthusiasm, culminating in what is called Pan-Celticism, has gathered around the recognition of this fact that the Irish, the Gaels of Scotland, the Welsh and the Bretons are Celtic peoples. ”
