To be honest this was the one thing on our vacation we felt was not really worth the effort. First - make sure you get a reservation since they sell out fast. It was very crowded, and the audio guide did not work very well – my wife could never get hers to go on her iPhone and mine worked but it is long, slow and really says basically the same thing on the signs. But the crowd is all jockeying for position to see/read the signs. Once you get to the book of Kells, you are not allowed pictures and you have to keep moving. They change which 2 pages you see every day and ours were VERY underwhelming: one page was blank and the other have a simple small picture. The library was amazing (but also crowded).
Just passing this along.