You have any thoughts on one of the fringe theories of the Phoenicians?
By 700 BC the "sea Celts" had arrived in Spain and were busy with a major invasion plan to take Ireland and use that as a base of operations to take Southern Britain.
The "sea Celts" had earlier ruled over and extracted tribute from the Germans along the Danube, but they'd been driven out in an uprising. The Greeks noted their arrival in the Black Sea as well as their departure into the Mediterranean.
BTW, the plan worked, the "sea Celts" took over Ireland and then Britain for the next half dozen centuries. They continue to dominate Ireland to this very day ~ somehow ~ since their languages still exist even if their special marker genes don't! They've become genetically identical to the Basque people.