One of the first things that Celtic illuminated manuscripts reminded me of were filigree designs and patterns. Filigree designs can be simple or get really detailed. LIke the illuminated manuscripts, they usually create repeating patterns using floral elements. Nowadays we see filigree designs in everything including jewelry, home decor, and stationary.
Wedding rings are a common place where you often see the use of the Celtic knot. Today you can find rings that have the traditional appeal of the Celtic knot or Claddagh ring incorporated with modern designs.
Lastly, I was reminded of designs you often see on scrapbook paper or journal/daily planners.