They came from an area known as Galatia along the Po River which is in the area of Poland. They traveled a lot and set up roots in not just Ireland but also NW Spain and Turkey.
The Letter of St. Paul to the Galatians is the letter to the Celts in Turkey.
According to anthropologists who have studied them, the Celts in Ireland are the closest we have today of the original Celts.
Paul’s letter to the Galatians is in Greek. It was addressed to Greek-speaking Christians in the Roman province of Galatia. Whether any of them also knew the Galatian (Celtic) language is unknown—the province included more territory than the area settled by the Galatoi.