Thomas stops Kimber from raping Grace by telling him that "she has the clap but then later explains the disease as Syphilis. The "clap" is gonorrhea not syphilis.
Usage of wooden matches doesn't begin until 1926. This story is using them 7 years before invented, and has them being used by a class of people that wouldn't be wealthy enough to own any for a number of years afterward.
The gun Freddy holds to Tommy's head is a Walther P38. It wasn't designed and developed until the late 1930's.
Before Tommy's meeting with the IRA men, the camera pans over the liquor bottles behind the bar of the Garrison Pub using stainless steel bottle pourers. However; the earliest patent of a 'bottle pourer', was submitted by Don A. Zimmerman and George W. Leventon in 1966.
Grace tells Billy "put on the Charleston". This is impossible as the Charleston was not popularized until 1923, and the scene was set in 1919.
Freddy Thorne is said to have received money from the Russian Embassy to support communist causes. The action takes place in 1919. The UK had no relations at all with Soviet Russia between 1918 and 1922, and formal ties were established with the (by then) USSR in 1924.
If there was a Russian Embassy in 1919 it was possibly operated by the Provisonal Govenment that was overthrown in October 1917. Hardly likely they would provide funds to support British communists.