The other day, someone asked where I thought the 2nd edition Celts Campaign Sourcebook might best fit in to a Greyhawk campaign. The introduction to the book promised suggestions for doing so, but this seems to have been forgotten by the time the book was finalized.
First, it’s important to remember that, even as presented in that book, the Celts are a barbarian people, with technology (such as arms and armor) below that of the “standard” AD&D setting, and this makes it somewhat difficult to import it as-is into Greyhawk. There are exceptions, such as the Suel barbarians in the Thillronian Peninsula, and the Olman folk in the south, but on the whole the assumption is that wherever you go in the Flanaess, you’re going to see pole arms, plate male, saddles with stirrups, and 9th level magic-user spells.
This doesn’t mean that the cultural/magical parts of the book can’t be applied, but I think it works best as this-is-how-things-were-in-the-past, rather than presenting things as they are in the current year. So, cultural attitudes, dress, and even class alterations can be applied.
This leaves the question of where, or to whom, the Celtic model best applies, although it must be emphasized that it’s a mistake to attempt to associate human races in Greyhawk (Suloise, Oeridian, etc.) with real-world races in all but the broadest terms (Baklunish are an analogue for Middle-Eastern/North African cultures, for instance, but to say they’re specifically Arab, or Persian, or Berber is going too far).
The obvious choice is the Oeridian folk, who are described as wearing checks and plaids (obviously associated with the Celtic peoples). The warlike nature of Oeridian society, as well as their organization along tribal/clan lines early in their history also points to their being well-suited to absorb some of Celtic culture’s mores, and some of the changes to game mechanics described in the Celts Campaign Sourcebook.
But as an alternative, I might point to the Flannae peoples, and their ancestors’ penchant for wearing body paint. They too are fierce warriors and were (and still are, in some cases) organized in tribes and clans, but with the same connection to the land as we see in the more Druidically-based culture described in the book. Again, much of the details in the book would apply to a more primitive state of affairs than we see in most of the Flanaess, but this would apply perfectly to the nomad culture of the Rovers of the Barrens, which is of Flan stock.