Interesting thought. I look very Irish, even so much so that was my nickname in high school and college. But short-pale, redheaded was very NW European anyway, and many think it came from Viking heritage, meaning when they invaded the British Isles long ago.
Yes, the Celts were all over EUrope, even in Hungary and Germany. They were eventually pushed out by the Germanic tribes as they too were pushed west and southward by the Mongols.
HOw did you find out Celtic via DNA? I am curious. My ancestor John and his friend Simon Dixon founded the Cane Creek MM about 1749/50. They are still buried there.