He was born in the 19th century on Earth and was a union man 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 finally gave the ultimate sacrifice for his union: he was found dead in the Allegheny River with 32 to 34 gunshot wounds in his body.
In 2372, Chief O'Brien encouraged Rom to form a union of bar employees to meet his brother Quark's unfair labor practices. He also told him about Sean O'Brien, but Rom did not find it a very encouraging example. (DS9: "Bar Association")