Includes correspondence, speeches, materials from political service and campaigns, vital and financial records, collected materials about legal and Irish history, and a diary from Joseph O'Connell's final year. Also includes materials relating to his legal career, such as court transcripts and briefs, correspondence about the American and Massachusetts Bar Associations, and newspaper clippings. O'Connell covered a variety of types of cases, including those involving Chinese immigrants, interpersonal disputes, murder, and a German saboteur in Maine.