The Captain, Jonathan Rankin O'Rose, was the twin, second son of an Irish merchant. Being the second son, with little hope of a decent inheritance, he was given schooling in hopes that he may one day start his own company. After the dissapearance of his older ( and somewhat reckless) brother, Owain, Jonathan was made heir to his father.
Unfortunately, this meant an arranged marriage to a mainland merchant's daughter. Not being the marrying type, Jonathan promptly joined the crew of an outgoing vessel, at the time not knowing that the vessel he had signed onto was the ship of an English privateer.
After hard work, a way with words, and some guile, he convinced the Irish and Scottish among to crew to remove the English captain. Now he was in control of the ship, and his own life.
Renaming the vessel the "Roisin Dubh" or the Black Rose, he and the crew continued their privateer ways... without reporting to the English government.