Jimmy Darmody was based on the real-life Jimmy Boyd, who was almost definitely not the Commodore's son. Boyd, who may or may not have been a Princeton dropout and World War I veteran, emerged as an assistant to Nucky during the bootlegging period, though not as early as the show suggests (1920). Initially a bellhop at the Ritz, Nucky's residence, Boyd rose up the ranks, becoming the leader of the Fourth Ward and unofficial overseer of all of Atlantic City's political wards.
When Nucky was deposed of power, Boyd was insulated from any charges and was an important bridge between the old regime and the new: that of Frank "Hap" Farley. Boyd, in addition to the head of the Fourth Ward, was also the clerk of the Board of Freeholders, the equivalent of a County Board of Supervisors. When Farley came into power, Boyd took a more important role in the Republican Party.
Farley wanted to remove himself from the extortion and racketeering that funded the party, so he left the illegal activities to Stumpy Orman, who collected from the brothels, gambling halls and numbers (illegal lottery) rackets. If someone tried to start a new venture without clearing it by Orman, he would close them down, controlling who the Vice Squad raided. On the other hand, Boyd was the COO of party affairs. He controlled patronage (political appointments), saw that public employees donated back to the party, and ensured that said employees were also active in donating their time to party causes. For a while, it seemed like the ward system was infallible.
While Boyd wasn't the type to stick-up bootlegging gangsters, he was a schemer who came up with ways to earn extra money. Because a city ordinance regarding boardwalk sales gave an advantage to veterans, Boyd teamed up with a veteran who would gladly "share profits" if he were the sole veteran licensed to sell ice cream. Boyd partnered with a Jewish lawyer who had connections to the ice cream supply in Philadelphia. The supplier agreed to work with just the endorsed veteran's enterprise. With everyone else locked out of the game or with costs prohibitively high, the monopoly flourished for years with a healthy chunk of the profits reverting back to Boyd.
The Efficient Drinker Blog has a more complete discussion of Boardwalk Empire characters and their real life counterparts.