One thing I haven't seen discussed much is exactly when and where "Celtic" coinage ceased to exist. The British celts issued coinage at least until the Roman conquest was complete, and most of the remaining "Celtic" kingdoms were conquered by the second century. Balkan celts issued a lot of imitations of Greek coins, but how long did these imitations last? Were imitations of Philip II tets still being made until Trajan came along? One interesting facet is that during times of societal breakdown (e.g. the Gallic empire and 4th/5th century turmoil) some of the local barbarous coins seem to have slid back to almost Celtic aesthetics: I bought these tiny (5-9mm) barbarous minims from a former collector of the type. I believe they are all copying Constantius II fallen horseman types. Once the Romans packed up and left Britain to the marauders, the earliest Anglo-Saxon sceattas are rather distinctively Celtic in design. Sadly, none to share as those are expensive little flecks of silver!
Finn235, Your question is a good one & there may be no definitive answer. One thing is for sure, when the Vandals, Visigoths, Ostrogoths, & Huns crossed the Rhine River they pushed the Celtic tribes as far west as possible (the French shore & Britannia). These invading Germanic tribes quickly adapted to Roman ways & unlike the Celtic tribes even accepted their own versions of Christianity. The German tribes made excellent imitations of Roman coins in circulation & their money passed freely as Roman money. There certainly were remnants of Celtic tribes that remained behind but their money in Celtic style only had value within their own tribes. The new German money gradually supplanted the Celtic money. Attached below is a German gold solidus I acquired about a year ago struck in the name of Emperor Zeno, late 5th - early 6th century, 4.48 gm.
Today's addition: Celtic, Coriosolites Stater C. 75-50 BC Grade: VF / aEF | Abbreviations Catalog: Castelin 211 Material: Silver/ Billion Weight: 6.16 g Size: 22mm