Most of what is known of the Celts before the birth of Christ comes from the records of Greece and Rome. But there appears to be many records. The Roman Emporer, (of history, not Shakespeare) Julius Ceasar, spent over twenty years recording his findings and observations of the Celts. He gained a fascination for them while an officer fighting the Celts in the Roman Legion. And his writings are still in print today.
The legends of Celtic fierceness are well founded and documented in ancient history. Rome was sacked three times, by the Celts, before its' completion. The Greeks built an entire city for the specific purpose of trading with the Celts, to stop their raiding of Greek possessions and territories. Romes' Legions built two walls across northern England to keep the Celts of Scotland and Wales from raiding Roman-held lands. With the advent of the construction of the second wall, the Celts joined forces and beat the Romans back beyond London. Apparently, the Celts felt the Romans were getting too greedy!
The discovery of Viking/Norse burial markers along the North American Great Lakes has show that Christopher Colombus was number two to reach the New World. Now, poor Chris is down to number three! Archeologists have recently discovered the ruins of a Celtic setlement in the American State of Mississippi!