Sean Aloysius O'Brien was a Human male who lived on Earth in the late 19th and early 20th centuries. Sean was a distant ancestor of Miles O'Brien, and by extension of O'Brien's children; Molly and Kirayoshi.
Sean lived and worked as a coal miner in Western Pennsylvania, and later became a labor union leader when Sean led his fellow coal miners during the Anthracite Strike of 1902, refusing to work and demanding their employers improve working conditions and benefits. His employers, unreasonable and disregarding their demands, forced Sean to lead a strike against the owners of the mines, making the mines close altogether, refusing to re-open them until their demands were met. Sean, a strong man at heart, kept the union together and kept the strike going.
The mines remained closed for 11 months, and the didn't open again until every last one of the miners' demands were met.
Sean never lived to see this day as, a week before the strike ended, he was assassinated by his employers and his body later found and was fished out of the Allegheny River, with 32 to 33 bullets in his body.
Sean, having finally given the ultimate sacrifice for his union, had the biggest funeral in all of Western Pennsylvania, having died more than a hero, but a union man, a man who gave his life to earn a decent wage for his fellow workers. His death made him a martyr, and thus his employers failed to end the strike in their favor.
O'Brien knew of his ancestor's example, and was full of pride at his example. O'Brien related the tale of Sean Aloysius O'Brien to Rom, when Rom could no longer stand the working conditions and unfair labor practices at Quark's bar on Deep Space 9, and decided to form a union of his own; the Guild of Restaurant and Casino Employees. O'Brien's words encouraged Rom (albeit the part of Sean's untimely death somewhat disturbed him), and allowed him to gain the courage to face opposition from Quark, and most especially the Ferengi Commerce Authority, where Rom rallied his workers back together after being intimidated by Liquidator Brunt, saying Brunt wouldn't have intimidated Sean, so Brunt won't intimidate him, and remained firm on improving their work conditions until he succeeded. (DS9 episode: "Bar Association")