The George D. Cahill papers contain correspondence to and from Cahill in his capacity as a local leader of several Irish nationalist organizations, including the Fenian Brotherhood, the Irish National Land League of the United States, and the Irish National League of America. The collection includes fliers, circulars, published speeches, and ephemera related to Irish causes, as well as membership and financial records for the organizations with which Cahill was involved. Correspondence and organizational records reveal the everyday activities of these organizations, especially of the Fenian Brotherhood and its ill-fated newspaper venture, the Irish National Publishing Company. The organizations' financial affairs, social events, and meetings are well-documented, as is their factionalism, infighting, and political maneuvering. Other Irish organizations such as the Clan-na-Gael, the United Irish Brotherhood, and the Combined Irish-American Societies of Massachusetts are represented in these papers, although more sparsely.

Notable correspondents include Patrick Andrew Collins, William M. Curry, Mary Francis Cusack (Nun of Kenmare), Thomas B. Fitz, Anthony A. Griffin, John W. Keogh, Francis F. Millen, Jeremiah O'Donovan Rossa, John O'Mahony, John Boyle O'Reilly, Joseph W. Paul, and George F. Train. The papers date from 1845 to circa 1921. Publications in the collection dating from after Cahill's death in 1893 may have been collected by the collection's donor, D. A. Brosnan, and also relate to Irish issues.