When Declan finds the gang in his home he threatens to call 'the Guards'. By this term he meant call the police. In Ireland the police are officially called the Garda Siochana, which most people shorten to Guards when speaking of them.
The futuristic looking silver car Sister Michael drives is the DMC DeLorean, famed for its appearance in the Back to the Future movies. The DeLorean was designed and built in the early 1980s with approximately 9000 cars assembled at a plant in Belfast, Northern Ireland. About 6500 of them are still in use.
The language spoken by the older woman the gang meets in Donegal is Irish. While Irish is taught in Irish schools the majority of the population of Ireland speaks English for everyday life. However, the Donegal region in the northwest has a significant Irish speaking population.
Conleth Hill (Carlos Santini) guest stars in this episode alongside Ian McElhinney (Joe Mcool) a series regular. Both actors also have recurring roles in Game of Thrones.
While it may seem odd that Sarah thinks she feels the presence of a dead American Air Force pilot in Derry, it is not altogether impossible. The port and area was a major Allied base in World War Two, with its large natural harbor used for receiving and assembling cargo ship convoys that traveled between the British Isles and North America. The region also had several air bases whose planes included American ones that flew long anti-submarine patrols in the area.