The long building is the Old Library whose primary function now is as the home of the Book of Kells. It also has a lovely giant hall, home to more than a quarter million old books, arranged in two floors of shelf after shelf (and not a trace of the Dewey Decimal system, from what I could tell). My sister and I took a Walking History tour of Dublin that started here (that's the group, the salt-and-pepper speaker our Guide, "A Trinity College History graduate")