Iron Age Celtic setting - my Ireland analog

[Ghostman]

Not all by any means. From the (admittedly little) I know, at around 500 BC, what we now know as modern Europe was inhabited by a plethora of different peoples of which the Celts were only one cultural type.

True that, though they were arguably the single most wide-spread cultural group (not a singular culture, mind you - far from it) at that time. Not that it's very easy to tell who were and weren't 'celts' at the time, due to scarcity of reliable evidence.

500BC was when Greece was in it's golden period.

Only if you fall in the popular trap of equating "Greece" with Athens. But this is going off topic...

(All this historical talk is to get you to understand the feel and circumstance, as this is not Western Europe, rather a complete fantasy heavily steeped in Celtic lore, technology and culture).

^ And that would be the most important statement to keep in mind for this discussion. GP doesn't have to limit his inspiration sources to western & central Europe of 500BC, even if he considers it a comparable analog. If he wanted to throw in some kilts and bagpipes, then he'd be perfectly free to do so, no matter how anachronistic.