Contains more than 1500 entries covering "individual Celtic countries and their languages, literatures, archaeology, history, folklore, and mythology.

"Geographically, as well as including the Celtic civilizations of Ireland, Britain, and Brittany (Armorica) from ancient times to the present, it covers the ancient Continental Celts of Gaul, the Iberian Peninsula, and central and eastern Europe, together with the Galatians of present-day Turkey, and it also follows the Celtic diaspora into the Americas and Australia."