O'Connell, Denis Joseph (1849–1927), bishop and rector of the Catholic University of America, was born on 28 January 1849 in Donoughmore, Co. Cork, son of Michael O'Connell, farmer and Bridget O'Connell (née O'Connell). When he was still an infant, his family emigrated to Charleston, South Carolina, USA, where three uncles and an aunt worked as missionaries. He attended the local schools and received additional tuition from his uncles. In the mid 1850s, the family moved to Columbia, South Carolina, and in 1865 moved again to Fort Mill, South Carolina. There O'Connell met Bishop James Gibbons (1834–1921), then vicar of North Carolina, who encouraged his vocation and became his mentor. In 1868, O'Connell entered St Charles College, Ellicott City, Maryland, to study for the priesthood. In 1871 he was sent to the North American College, Rome to study theology at the Urban College of the Propagation of the Faith. He earned a doctorate in sacred theology there and was ordained on 26 May 1877.
While O'Connor was in Rome, Gibbons was transferred to the diocese of Richmond. O'Connell's first assignment was as assistant pastor of St Peter's cathedral in Richmond; however, he shortly returned to Rome where he acted as a postulator for Gibbons when he was appointed coadjutor bishop of Baltimore. In April and May, O'Connell accompanied George Conroy (qv), bishop of Ardagh and apostolic delegate to Canada, on a visitation tour of the USA. In 1883 Gibbons requested O'Connell's transfer to the Baltimore archdiocese. He accompanied Gibbons to Rome. When the council met in Baltimore in 1884, Gibbon served as one of its secretaries. At its conclusion, he returned to Rome with an American delegation of bishops seeking Papal approval for the council's decrees.
In 1885 O'Connell was appointed rector of the North American College in Rome, the residence for American seminarians. As rector, he supervised the expansion of the college's facilities and services and increased the number of residents. He was appointed a domestic prelate in 1887. He also served as the unofficial ambassador to the Vatican for the US bishops and for Cardinal Gibbons and promoted the liberal Americanist agenda of archbishops Gibbons, John Keane (qv) and John Ireland (qv), which addressed important contemporary issues such as the relationship between church and state and the social conditions of workers. When the conservative wing of the US hierarchy gained power in 1895, O'Connell lost his position as rector but remained in Rome until 1903 as rector of Santa Maria, Gibbon's titular church in Trastevere, and continued to promote liberal Americanist views. In an address before the August 1897 International Catholic Scientific Congress in Fribourg, Switzerland, he argued for the separation of church and state as a way for American catholics to participate actively in secular society; the Americanist cause was lost, however, when Pope Leo XIII rejected their position.
When O'Connell returned to the US in 1903, he was appointed rector of the Catholic University of America. Despite some initial success in increasing enrolment and recruiting a quality faculty, his authoritarian manner alienated the faculty and the board of trustees. O'Connell also had to work to restore fiscal confidence after the treasurer of the university misappropriated institutional funds.
After O'Connell was raised to be a titular bishop on 12 December 1907, Archbishop Patrick W. Riordan (1841?–1914) of San Francisco requested his appointment as auxiliary bishop, which occurred on 25 December 1908. In 1912 O'Connell returned to Richmond as bishop. Poor health forced his resignation on 15 January 1926, and he died 1 January 1927. He is buried in Richmond.