It's not abstract: that would imply that it was not based on any real form. But it is not intended to be realistic. We know a lot about Celtic art (https://en.w

It's not abstract: that would imply that it was not based on any real form. But it is not intended to be realistic. We know a lot about Celtic art (https://en.wikipedia.org/wiki/Celtic_art https://en.wikipedia.org/wiki/Celtic_art) and it often shows very high skill, and is deliberately non-realistic. That's hardly surprising: historically realistic art has been in the minority, with most cultures choosing to show some aspect of a subject selectively. Let me give one familiar modern example: logos. If they are graphical, generally you need to put some work in to understanding what they mean.