the O'Sullivan family line once ruled county cork, until the notorious potato famine, many of the O'Sullivan family emigrated to Manchester for a better life, although just how many and exactly when is a mystery.
As with any name starting with O followed by an apostrophe, it means 'Brother of' or even 'Slave of' So the original O'Sullivans would most likely have been freed slaves or nameless families who where accepted by the Sullivan family as their own for great deeds or services rendered.
Famous O'Sullivans include Ronnie O'Sullivan (snooker player) and Richard O'Sullivan(1960's actor, Robins nest)
They are generally prolific breeders and have been known to father children out of wedlock, or simply deny the existence of an offspring/sibling for reasons known only to themselves, Also notable is an inherent tolerance for alcohol, probably stemming from Irish roots, most O'Sullivans enjoy Guinness, however, this is not as common any more, most will just drink until their vision is impaired and their legs unable to transport them directly to their destination and yet somehow wake up right as rain.
OH do you know Ronnie?
Friend: Run away before he gets our potatoes.