I believe the Celts were all in through that country. The people the Apostle Paul wrote to in his Letters to Young Churchs were largely Celtic.
Celts captured Rome except for the Capitoline hill in 387 B.C. and occupied the Po Valley in Italy for centuries. In the 3rd century B.C. some of them invaded Greece. Eventually they settled down in Galatia in Asia Minor. St. Paul traveled through there (Acts 16.6; 18.23) and wrote a letter to the Galatians--but the letter is in Greek and there is nothing to show whether the recipients were of Celtic ancestry. (They could be descended from Greeks who had settled in that area, or of other non-Celts in the Roman province of Galatia who had become Greek-speakers.)
Probably there were many similarities between the customs of the Thracians and the ancient Celts. Herodotus said that among the Thracians "to live by war and plunder is of all things most glorious."