The Irish Republican Army has given up its armed struggle for a united Ireland, agreeing to turn solely to political methods, an American businessman said yesterday after being briefed on a statement expected from the guerrilla group later this week. The agreement, if borne out, would be a historic turning point in the violent history of Ireland and Northern Ireland. But there is still widespread official skepticism about I.R.A. promises, particularly when it comes to the issue of disarmament. Full Story