WJD MON 19 DEC 2011
The main difference is linguistic, Brythonic speaking vs. Goidelic speaking or P-Celtic vs Q-Celtic, the Goidelic speaking Celts represent an earlier migration from their central European homeland, the material culture is similar and their beliefs are largely the same, there is some differentiation in their burial practices.
The Gauls and Britons are Brythonic or P-Celtic speakers, in the modern world the surviving languages in this family are Welsh and Breton. The Celt-Iberians and Irish are Goidelic or Q-Celtic speakers, surviving branches of this language family are Gaelige (Irish), Scots Gaelic and Manx (resurrected). None of them are what I’d call healthy languages, Welsh is probably doing the best.
Related to these people are the Ligurians, sometimes called the Celto-Ligurians, from south eastern France/north western Italy, their language and culture were similar to the Celts, and the Romans and Greeks considered them, at least sometimes, to be Celts, at other times to be their cousins. Also, the Picts, not much to go on, just a few personal names and place names viewed through the lens of other languages, some scholars believe they, along with the Ligurians, may have represented a third, even earlier migration from the Celtic heartland. The Picts, for their part, after centuries of maintaining their own identity separate from the Britons, lost all trace of a separate identity within a couple of generations of being absorbed into the Gaelic Kingdom of Alba, which later became known to us as Scotland.
Most of the Stone circles are along the Atlantic seaboard or in the British Isles, there exist similar phenomena all over the world though, including China and North America, just not on as grand a scale. There are also extant wood henges, or at least the archaeological remains of them, so perhaps they were designed as temporary gates?.