6. Trinity College Library, Ireland
When completed in 1732, the library at Trinity College in Dublin had a flat plaster ceiling; but as its prodigious collection grew, expansions were required. In 1860, the roof was raised to allow construction of the present barrel-vaulted ceiling and upper bookcases. Thousands of rare and very early books and artifacts are housed here, including the "Book of Kells" and the Brian Boru harp, a medieval Gaelic harp from which the national symbol of Ireland was derived.