Did Margaret Schroeder Have a Real-Life Counterpart?

In a word, sort of. A Margaret Schroeder-type appears in the Boardwalk Empire book's prologue as an anonymous,  housewife and boardinghouse maid who is upon hard times and requests to see Nucky Johnson. She had previously met Nucky just once, at her father's wake and this time explained how her husband, a part-time baker helper, has gambled away the family's food money. Nucky gives her money, arranges a ride home for her and promises her husband will be banned from local gambling halls. So, this part closely follows Margaret from the first episode.

Was she an Irish immigrant? Maybe. Did Nucky arrange to have her husband killed and later have relations? Almost definitely not.

In the book's source notes, this lady is revealed to be Mary Ill, a woman who was active in local politics and charitable organizations -- Temperance Union perhaps? She maintained a friendly relationship with Nucky and helped him remain influential within her circles.

Who are Leo and Ignacious D'Alessio's Real-Life Counterparts?

Leo and Ignacious are somewhat based on two of the six Lanzetta Brothers, 1920s Philadelphia gangsters. Leo was the leader of the family and Ignatius (spelling difference from Boardwalk Empire character) were heavily involved in the numbers racket, as well as bootlegging.

Their bootlegging was of a different variety than the typical Atlantic City method of bringing it in by sea. The Lanzetta brothers would provide people who lived in tenements with the raw materials to distill liquor and then buy the finished product. With this method, the brothers remained impervious to prosecution since the "Alky cooks" wouldn't rat on them if caught. Known for their ruthlessness, the threat of violence was enough to keep the brothers one step from the law in such instances.

How Real is Chalky White?

In a word, not very. There may be some basis as Michael K. Williams (Omar from The Wire) alludes to the fact that the character is based on a boxer. One would have to think that was "Chalky" Wright, a boxer who fought 200+ matches in a long career that spanned through the 20s, 30s and 40s. However, Wright fought few matches in New Jersey and once fought a match as "Chalky White," probably due to a clerical error.

Wright fought out of California, primarily Los Angeles and San Bernadino in the 1920s, so the HBO character seems like a pretty loose interpretation if this is who it is based on. He fought often enough that his involvement in bootlegging was probably peripheral (hired muscle), if anything.

More on which characters are real in Boardwalk Empire

The Arnold Rothstein Poker Scene

This scene was really weak. Do the writers and directors really have no concept of poker? I guess the game was played differently back then, but some parts of it were the same, especially to an odds-man like Rothstein.

The first annoying part was the string raising. I guess that it was allowed back then, but it's hard to know. Now, you fold, call or raise. You don't "see" someone's bet (call), gauge his reaction and then decide to either stay with the call or throw in a raise. It's been that way for a while, but given this takes place  in between when Maverick was set and present-day, who knows what the table culture was like back then.

The worst part was the distributor's raise.

Is Jimmy Darmody Based on a Real Person?

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.