Born in County Cavan, Ireland, O'Connell sailed to San Francisco upon the request of Bishop Alemany for priests to serve in the diocese of Monterey.

In August 1850, Joseph Sadoc Alemany, Bishop of Monterey, California stopped by All Hallows on his way back from Rome, looking to recruit priests for his new diocese.

He arrived in San Francisco on July 1, 1851 and was appointed pastor of Mission Santa Inez, where he was also director of the diocesan seminary.

When Pope Pius IX declined, O'Connell responded, "“You mean I am condemned to the mines.”[4] He was consecrated titular Bishop of Flaviopolis, and Vicar Apostolic of Marysville, 3 Feb., 1861, at All Hallows College, by Cardinal Paul Cullen, Archbishop of Dublin.

[3] O'Connell reached Marysville 8 June, and was inducted on the following day at St. Joseph's Pro-cathedral by Archbishop Alemany.

[2] In 1868 the vicariate was erected into the Diocese of Grass Valley and Bishop O'Connell was transferred to this title 3 Feb. of that year.

Located in northeastern California, United States, the diocese also included most of Nevada,[5] and, early in its history, Utah and part of Colorado.

[2] The remains of Eugene O'Connell were moved from Los Angeles to Sacramento over a hundred years after his death.