The Celts and Druids didn't invade Scandinavia because?
Roman influence?
The Celts didn’t need to conquer the Viking by invasion - they did so (in a way) through the kidnappings. The Vikings favored the lovely hardy red-haired lasses they encountered and took them home to breed... many Vikings had Celtic mothers and were raised by them, not their absent fathers.
So I’ve been told. :)