Celt is a historical term used to refer to a group of Tau'ri from Ireland, Scotland, the Isle of Man, Wales, Cornwall and Brittany. In the past, so Goa'uld took several Celts to other planets to enslave them. There are now descendants of the Celts on these planets as Volian, Hebridians and the people of Newgrange. (SG1: "Forsaken")
Nuada and Cernunnos were two of the "gods" that ruled over the Celts. (SG1: "The DVD Collection 13", "The DVD Collection 19")