Disowned Adaptation: Tom Clancy was very unhappy with the numerous changes the film made from the original novel. He also felt that the 49-year-old Harrison Ford was too old to play the 31-year-old Jack Ryan.
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).
What Could Have Been: 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.