Joyce found his wave thunder (here)

In all the places I have been to I have taken ((The Book of Kells)) with me, and have poured over its workmanship for hours. It is the most purely Irish thing we have, and some of the big initial letters which swing across a page have the essential quality of a chapter from Ulysses.

-- James Joyce to Arthur Power in Ellman’s biography "James Joyce," p. 545