I always thought all the Celtic animal art and letter art that is all interconnected came from the Celts, but no, I was wrong, all of that art comes from the Viking influence and the knot work art appears after the Viking invasion of 800 AD. And moves on to 1150 AD.
I’m afraid that’s not right. The traditions of indigenous Northern European art are very complex. What many people today consider “Celtic Art” is in actual fact Insular Art, which is a combination of Celtic, principally Irish/Gaelic, and Anglo-Saxon art, for this reason it’s also known Hiberno-Saxon. Nevertheless, the Celtic elements in this art can be traced far further back to La Tène styles, which themselves blended ancient traditions that may have come down from the late Stone Age with later elements of Greek and Etruscan art – peoples with whom the Celtic peoples traded.
Naturally, all of these ancient peoples traded with each other and influences spread, much like the famed knotwork of Insular Art, they interlace and weave intricately, so you can find elements combining that come from as far afield as Scythian territory and southern Russia, the Mediterranean and the Middle East.