The O'Brien family is a family of Humans originating from Ireland on Earth. The O'Brien family tree can traced as far back as the Irish king Brian Boru. At some point the family emigrated to the United States of America. Sean Aloysius O'Brien was one of the earliest known members of that family, who lived in Pennsylvania in the United States of America in the late 19th and early 20th centuries. (DS9 episode: "Bar Association")