I accidentally looked up Celtic knots last night. Don't do that. It's a pretty amazing graphic design (and math?) rabbit hole. Also hey, Insular Art, the Book of Kells (the movie Secret of Kells is about a real book, I did not realize that), and the Picts, who seem to have directly inspired both Skyrim's Forsworn and Terry Pratchett's Nac Mac Feegle.

I found this software http://knotter.mattbas.org/Knotter which is actually a really good starting point for the more regular knots. Where it appears to fall down is that the curves are not smoothed, so you can get a rough circle out of it, just not a smooth one, and getting to do complicated things with mixed weights sounds more difficult than drawing it myself. But it's very strong as a playground and as a place to sketch your design, from there bring it into illustrator or draw it by hand...

By taking the output and slicing it up into a tile and putting it into a mesh I was able to get what I was looking for.

(need a better bg tile obv)

...and I think I better just call it a win and stop right there. I don't want to get lost. To that end I've decided that every time I see the word "Gaelic", I'll shift a letter and internally read it as "Garlic." That should help things.