In compiling this authoritative dictionary, Fr. Woulfe spent 25 years in the field collecting names, communicating with native Irish-speakers, and studying the different forms of Gaelic. The work begins with an illuminating history of Irish names, followed by the great dictionary–700 pages of Irish names in two arrangements: English-Irish and Irish-English. The English-Irish section contains 3,500 Christian names and surnames, each dealt with as a separate group. The Irish-English section, containing over 7,000 entries, gives the original form of the surname and its precise meaning, then its English equivalent and the former and present location in Ireland. Besides personal names, there is a section on Irish clan names, which are of great importance in tracing the early history of Irish families.