What does advanced mean objectively? Or to you?

Very hard to say.

There are significant gaps in almost all the celtic histories and entire cultural groups like the picts who we know almost nothing about.

In addition the few "histories" we do have like the tain and the book of invasions are so heavily mythologized that it's difficult to say what they were even based on.

Sorry this isn't more help - an archaeologist might be able to help more.

Most accounts from back them are written by romans which means the celts didn't really read or write but there are accounts of celtic tribes that were really skilled and advanced in smithing and mining and they also had large towns