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.