One of my two museum visits in Dublin was The Long Room at the library at Trinity College. I've been deeply keen to see the Book of Kells for some 20 years (when I took a wonderful calligraphy course at Reed College). I was, however, deeply disappointed in how Trinity presents and displays the wee bits of their tremendous collection that they in fact share. The Book of Kells itself wasn't on display the day I was there, but based on the miserably lit and weakly labelled texts that were on display, I'm not sure I missed much.
All that said, The Long Room was visually remarkable. For a book junkie, and especially an old book junkie, all that wood and leather was absolutely delicious. A pretty crap display space, however.
My second museum visit that day was to the Chester Beatty Library, which was a vastly better museum. They have a phenomenal collection of early, early manuscripts, and their display of the material was much, much better.