Technology Marches On: The British police investigating the attack on the royals are quite impressed by Jack's "Cambridge Datamaster Model-C Field Computer", which has two megabytes of memory. Well, it is pretty impressive for 1983.
Backed by the Pentagon: The CIA scenes were filmed at the actual CIA headquarters; this was the first time the CIA had ever done such a thing.
Fake Irish: Only three actors in Patriot Games were actually Irish: Richard Harris, Patrick Bergin and Jonathan Ryan. This means all of the other Irish characters were this. Englishwoman Polly Walker plays the IRA terrorist Annette, though it is revealed in film that Annette is British-born (presumably of Irish ancestry).
Missing Trailer Scene: The line "There's never been a terrorist attack on American soil" was included in trailers for movie, but was left out of theatrical release because it sounded too much like an invitation or dare.
Old Shame: Alun Armstrong is said to be embarrassed by his involvement with the film.
The Troubles largely ended after the 1998 Good Friday Agreement.
One scene hinges on the Ryans communicating by car phone, which was a pretty high-end item in 1990.
On a meta level, Sean Bean's noodle arms, highlighted by a series of ill-fitting t-shirts, would only be acceptable in a big-budget movie prior to the 2000s, when physical training became a staple for actors playing heavies.
The final battle was originally supposed to have Ryan and Miller fighting on large rocks in the middle of the stormy sea. The fight would end with Miller drowning. Test audiences weren't excited with this conclusion, so the final battle was changed to a boat chase instead.