"Gaelic" is not an ethnicity, but a language. It is one of the ones belonging to the Celtic Culture. Celtic in modern day terms refers to only Scotland, Ireland and the Isle of Man in the Q-Celtic, and Brittany, Cornwall, and Wales in the P-Celtic divisions.
Studying on Celtic culture would take a student back to the days of Greek culture, since it was them who came up with the term Keltoi from which the Celtic name for the culture is derived. The Greeks knew them, the Romans fought them (branch known as the Gauls). Both Greece and Spain have their own Galacia's which are Gaulish communities. See National Geographics, September 1977, I believe.