In 1840 Prendergast was commissioned to make some pedigree researches in
County Tipperary, which led him to a study of the settlement of Ireland at the restoration of Charles II, and also of the Cromwellian settlement. His researches culminated in
The History of the Cromwellian Settlement of Ireland (1863, 2nd edit. 1875). (1868, Dublin, 2 pts.). In 1881 he prefixed a notice of the life of
Charles Haliday to the latter's
Scandinavian Kingdom of Dublin, and in 1887 he published
Ireland from the Restoration to the Revolution. Prendergast was also an authority on Irish pedigrees and archæology, contributing, among other papers, to the old Kilkenny Archæological Society's
Journal,
The Plantation of Idrone by Sir Peter Carew. In articles published anonymously in the Dublin press (1884–90) he covered local knowledge of the old houses of Dublin. In politics he was a liberal, contributed to the old
Nation newspaper, and replied there in 1872–4 to
James Anthony Froude's lectures in America on Irish history. He authored numerous pamphlets. He opposed Home Rule, and from 1878 he was an opponent of
Charles Stewart Parnell. ==Legacy==