Born on June 28, 1844 in Dowth Castle along the River Boyne, O'Reilly was conscripted into the British Army as a young man. He was later charged with sedition against the British Crown and sentenced to life imprisonment in an Australian penal colony. O’Reilly made a daring escape aboard a New Bedford whaler, Catalpa, in 1868, a feat that helped shape his legend by the time he landed in America.
Arriving in Boston in 1870, he spent the next 20 years reconciling the city's racial and ethnic factions who struggled against one another. He became editor and then owner of The Pilot, the leading Irish Catholic paper in America, using the paper as a bully pulpit to advance various causes. He befriended the Yankee establishment while admonishing them for the prejudices.
O'Reilly defended American Blacks who were still looking for post Civil War equality. He welcomed new immigrants such as Italians, Jews and Chinese, insisting that they get the same privileges as nativist Americans. Throughout his life he pursued freedom of Ireland from Britain, advocating for home rule and land reform.
In 1885 he delivered a thunderous speech in defense of the rights of Black citizens at Faneuil Hall before the Massachusetts Colored League. He said "So long as American citizens and their children are excluded from schools, theaters, hotels, or common conveyances, there ought not to be among those who love justice and liberty any question of race, creed, or color; every heart that beats for humanity beats with the oppressed."
O'Reilly was a popular poet and speaker, often called upon to deliver poems at noteworthy occasions such as the unveiling of the Boston Massacre Monument on Boston Common in 1888. There, he read a poem dedicated to Crispus Attucks, killed by British soldiers at the Boston Massacre of 1770. Attucks' father was an African slave and his mother an American Indian.
O'Reilly died on August 10, 1890 from an accidental overdose of medication. He was taken back to St. Mary's Church in Charlestown for the funeral, one of the largest in Boston's history. "The greatest of Irish-Americans" is dead, proclaimed The Pilot.
A memorial to O'Reilly was commissioned to noted sculptor and friend Daniel Chester French. Vice President Adlai Stevenson and hundreds of Boston's prominent and ordinary citizens attended the official unveiling on June 20, 1896. The bust of O'Reilly is set against a Celtic design stone, and the back of the memorial has bronze allegorical figures of Erin, flanked by Poetry and Patriotism. The O'ReillyMemorial is located in Boston's Fens at the intersection of Boylston and Fenway Streets, and is part of Boston’s Irish Heritage Trail.
O'Reilly lived at 34 Winthrop Street in Charlestown, where there is a plaque in his honor. In 1988 the city dedicated a plaque to O'Reilly in Charlestown at Austin and Main Streets. His summer home in Hull is today the town's public library.
O'Reilly is buried at Holyhood Cemetery in Brookline, Massachusetts.