Gathering the Tools
It's amazing the difference technology makes! When I started drawing Celtic art, the reference material available was minimal. Now, the great libraries of the world make it easy to view their collections online. I'm not sure when this began, from around 2010 the quality of online reproductions has been excellent.
Trinity College, Dublin, has shared the Book of Kells and other early Irish manuscripts with the world. The British Library has wonderfully high resolution images of the Lindisfarne Gospels and other 8th-9th century works.
Other British and European libraries and museums also have excellent images of manuscripts, carvings, and artefacts available to view online. I still lose hours looking at a single photo, but I've been able to expand my knowledge of traditional knots and forms far beyond the simplified fragments and low-res images that I started with.