An ancestor of Miles O'Brien who led Pennsylvania coal miners in the anthracite strike of 1902 and was murdered for it, the example Miles invokes when Rom unionises Quark's bar.
Sean Aloysius O'Brien was a coal miner and labour organiser in Pennsylvania at the start of the 20th century and a forebear of Chief Miles O'Brien. In 1902 he led the anthracite miners out on a strike that lasted eleven months before the owners gave in. He did not live to see the victory: he was found in the Allegheny River with thirty-two bullets in his body.
Miles tells the story in the Deep Space Nine episode Bar Association, when Rom forms a union of Quark's employees and the station's officers take sides. Bashir suggests the ancestor was a hero; O'Brien answers that he was something more, a union man. In the same conversation he claims descent from Brian Boru, the medieval king of Ireland.
Sean O'Brien is never seen, though his example hardens Miles in support of the strikers, to the point of a brawl with Worf.