This work is the magnum opus of Irish genealogy, a vast and prodigious compendium of family history and source material. The first volume explores the origin and stem of the Irish nation. The old Irish genealogies assembled here are brought down to the lineal descendant of each family living at the time of the British dispossession, although many of the descents are brought down to the 19th century. Also included is a lengthy appendix with an extraordinarily detailed table showing families that owned land in the 12th century. In addition, there is an index of several thousand surnames.
The first half of Volume II consists of Anglo-Irish genealogies, all carried down at least to the Commonwealth period, and most to the last quarter of the 19th century. Arranged alphabetically by family name, these hundreds of genealogies are heavily annotated, and being supported by references to events of comparatively recent history, they sometimes trace the line of descent to an American branch of the family. There also is data on the Huguenot and Palatine families of Ireland and a chapter on the Ulster Plantation and Scots settlers. The latter half of Volume II is encyclopedic in coverage, bearing reference to countless persons, places, and events associated with Ireland.