The Outsider (1979)

Michael Flaherty (Craig Wasson), an American Vietnam veteran of Irish descent, returns to Belfast to join the cause of his grandfather, Seamus (Sterling Hayden). Soon he finds that he is not as welcomed in his home country as he imagined he would be. Even worse, he's the target of an IRA assassination plot designed to make the British forces look bad in order to elicit financial support from wealthy Americans.