Considering that the cradle of humanity was likely the African continent, it's not unreasonable to believe that the original Celts were darker people thousands of years ago. Adaptation, though not necessarily evolution (since that takes a few hundred thousand years), eventually made them lighter skinned and lighter haired, in harmony with their environment.
Take Michelle Borth for instance. While she is part Italian, that exoticness about her is not from Italian side. Italians are whiter than the Celtic nations. Italians have the Scandinavian invasions. Plus the Romans. I believe this is coming from her Scottish side, which begs me to ask this question.
No, the original Celts were not darker people, because they have not changed much over many generations. There is no evidence that they have changed a great deal over the history of their people. Without that evidence there is no reason to believe that they are much different than they were years ago.