Episode #1.3 - S1-E3
Factual error: When attempting to arrest Grace one of the supposed IRA men proclaims that he is taking her into custody "on behalf of the Irish Free State." The term "Irish Free State" was not a term used by republicans or generally until the Anglo-Irish Agreement which led to the end of the War of Irish Independence and the establishment of the Irish Free State in 1922. Anyone representing the IRA would have, when speaking English, referred to the Irish Republic at that time.
Episode #1.3 - S1-E3
Factual error: One of the men who approaches Tommy claiming to be representatives of the IRA sings the song "The Boys of the Old Brigade." The song is told from the perspective of an elderly veteran of the Irish war of independence which had only began in January of 1919 which had happened only 3 years before. The song was written by Paddy McGuigan of the band Barleycorn in the 1970's.