Dawson Casting: Beatie Edney played 17-year-old Carol Richardson while at least 30 at the time of filming herself (although she's later aged up as her character spends 15 years in prison).
Fake Irish: Daniel Day-Lewis, Pete Postlethwaite and Mark Sheppard. However, got Day-Lewis got Irish citizenship and lives there, while Sheppard has some Irish heritage. All of them put on good accents too. Mostly it's averted anyway.