I hope this adds some understanding to this arguement. First of all the Western Tribes of the Celts were dark haired, Eastern and Northern Celts were lighter, mainly because of intermarraige with Germanic tribes. The Celtic tribes that settled Ireland came from the Western Celts, ergo the dark hair. Further colonization of Ireland by Vikings and Germanic Tribes from England added some variation to the theme, hence the lighter shades and the red heads. In Northern Spain today, you'll see the original settlers of Ireland, and they have some light haired people as well, as they mixed with Germanic Tribes. As you mentioned, the Romans had many people fighting in their army, most of their calvary were Eastern Celts and Germans.