Even the term "Celt" can be misleading when applied to "Tradition". Exactly how extensive the Celts really were, and their historical origins are hotly debatable. The standard accepted view that the Celts emerged in Central and Eastern Europe, and spread Eastward as far as the Carpathian basin, and Southwards into the Balkans and Anatolia, is only tenuously supported by evidence. There is no archaeological or textual evidence that supports any Westerly Celtic migration at this time (6th-4th Centuries BC) But the earliest Greek and Roman texts mention that the Celtic language was spoken in the South West of the Iberian peninsula as early as the 8th century BC.
By the C6th BC, when classical sources begin, there were Celtic speakers in Iberia, Gaul, and (possibly) Britain & Ireland, which suggests the origins of the first Celts were on the Atlantic seaboard of Europe. The later, and well documented Celtic diaspora Eastwards around 400 BC only applied to the inland Eastern and Central European Celts, not the Atlantic communities.
So if the thesis of the Westerly origins of the earliest Celtic culture and tradition is to be accepted, and with the premise that language is the unifying factor for the ethnic origins of an identifiable culture, then the origins of all things "Celtic" actually has it's roots deep in the prehistory of the earliest European Atlantic peoples, and not in central and Eastern Europe at all. Which kind of throws a spanner at the veracity of all Celtic "tradition", as posited by the reconstructionist body of mainstream Celtic Historians.
So it goes. Much of what is held to be Celtic tradition, may actually be inherited from Mesopotamian sources, and was picked up by the Eastern periphery of Celtic peoples due to their Geographical proximity to these earlier cultures at the time of the first classical references to the Celts.
And the roots of genuine Celtic tradition may very well have been formed half a continent away, and be yet to be empirically examined to any great extent. Food for thought, isn't it?