During his early career he served in the California State Legislature and as a member of the Nevada constitutional convention.
Details of Dunne's education are unknown but the judge's broad background in history and theology combined with fluency in multiple languages indicates extensive schooling.
[4] During his youth, Dunne was a Douglas Democrat but joined the Union Party following the outbreak of the American Civil War.
A later trip, in 1874, resulted in Dunne meeting with historians who determined he was "the legitimate legal heir of Iregan and chief of his tribe".
[5] The marriage produced five children, including respected Maryland judge, Eugene O'Dunne, and longtime Santa Fe society gossip columnist and author, Brian Boru Dunne (1878-1962).
During his spare time he traveled throughout the territory with Governor Safford and was credited with planting Arizona's first orange orchard in late 1874.
Examples of his rulings include Grounds v. Ralph, 1 Arizona 227 (1875) which rejected an appeal by a justice of the peace on the grounds the appeal did not specify the nature of the supposed legal error and was for an amount less than required by statute, Ford v. Haynes, 1 Arizona 229 (1875) which determined a debtor could show preferential treatment to a creditor prior to filing for bankruptcy, and Thorne v. Bowers, 1 Arizona 240 (1875) which determined an intermediary who obtained a larger sum for a mining claim from a purchaser than agreed upon by the seller could keep the difference between the two prices.
The Chief Justice remained on the bench until the end of his January 3–26, 1876 court session,[19] and his successor was sworn in on February 1, 1876.
The next year he was living in Santa Fe, New Mexico and in 1878 moved to Utah where he served as legal counsel for a mining company.
[20] Dunne gave the commencement address at Notre Dame in 1880[21] and was living in Chicago when he became involved with Hamilton Disston's 1881 efforts to purchase roughly sixteen percent of peninsular Florida.
[24] In 1976 the town's bicentennial commission placed a marker in the northwest corner of the plaza recognizing Dunne's contributions in establishing the community.
In 2010 the Diocese of St. Petersburg added a Catholic Heritage Marker to the north facade of Saint Anthony of Padua Catholic Church in San Antonio, recognizing both Dunne's and the parish's part in the 19th century church history of Florida's West Coast.
His final two years were spent fighting an illness from which he died at St. Agnes Sanitarium in Baltimore, Maryland on October 4, 1904.