The Library was one of the first to implement the Dewey Classification System. The main part of the printed books collection catalogued prior to 2001 consists of two separate sequences classified by Dewey number. The Irish Collection (with the prefix Ir) also includes the sequence of bound volumes of pamphlets (with the prefix P) is stored in the main library building. It consists of books by Irish authors on all subject matters, and books on Irish subjects, published in Ireland and abroad, up to the year 2000.
All books catalogued since (except reference books and those for Special Collections) are added to a single ongoing collection, with a different numbering sequence. This consists of three elements – a digit indicating the year of cataloguing; a letter (usually A or B) indicating the size of the item; and a (sequential) running number. Thus, the first book in the new sequence has the number 1 A 1. All of this sequence is stored in the main library building.