He was born in the 19th century on Earth and was a union leader in Pennsylvania in 1902. Sean and a group of employees went out on strike against the owners of a coal mine and refused to open the mine until their demands were met. He made the ultimate sacrifice for his union: he was found dead in the Allegheny River with 32 or 34 gunshot wounds in his body a week before the strike ended.
In 2372, Chief O'Brien told Rom about Sean O'Brien, after Rom formed a union of bar employees to fight his brother Quark's unfair labor practices. Rom remarked that this was not a very encouraging example. O'Brien was proud of his ancestor, as he "was a union man". (DS9: "Bar Association")