I realize that the show is clearly fictional and sensationalized, but I'm curious as to what post-WWI organized crime would have looked like in England and whether the scale of crime and murder was anything like what the show depicts.
I'm going to try to recount the murders shown in the first season, so that maybe some folks who have historical knowledge, but haven't necessarily seen the show can comment.
Thus, I warn you, spoilers ahead:
The basic premise, is that a decorated soldier returns home from the First World War, assumes control of his family's gang, and begins to try to improve the gang's in the world rather aggressively. A major plot fulcrum in the first season is that some of the gang's men have bungled a burglary accidentally stealing number of machine guns and other arms and a large quantity of ammunition from a local armory. The government sends a member of special branch to retrieve them, who turns out to be rather ruthless fellow fresh from northern Ireland. He, among other strategies, sends a female spy into the neighborhood in the hopes of infiltrating the gang.
It's not clear to me exactly what time frame the season covers, but I think it's several months. In that time, I can recall the show depicting seven homicides, but if someone else remembers one or more that I missed, please don't hesitate to jump in and add it.
One Italian immigrant is killed by a local man, who is not then a part of any gang, when he experiences an episode of PTSD.
One member of the IRA is killed by informant, who is not targeting the IRA, but happens to despise that group.
Two members of the IRA or killed in the course of trying to make a deal with the gang. Everyone is trying to doublecross everyone, and the informant is again involved. A sting goes wrong, and the informant and the gang leader have to kill the two IRA members who are attempting to kill them.
One member of the gang and the leader of a rival gang are shot at the outset of what seems likely to become a major gunfight. Because the second person killed is the leader of the rival gang, the battle is avoided as his forces disperse.
And lastly the informant is killed by her handler, basically over romantic tension, though there are of course tinges of honor, duty and betrayal of country.