At a trial in Ireland’s special antiterrorist court, an F.B.I. informant offered new details today about the guerrilla groups that have broken away from the Irish Republican Army and destabilized Northern Ireland’s fragile peace effort with terrorist attacks here and in Britain. The informant, an American truck driver named David Rupert, is the main prosecution witness in the trial of Michael McKevitt, the man charged with running the Real I.R.A., a breakaway guerrilla group. The Real I.R.A. was behind the 1998 bombing in Omagh, Northern Ireland, that killed 29 people and maimed dozens more. In the no-jury court, Mr. Rupert told a panel of three judges that he and Mr. McKevitt discussed fund-raising in America, procuring weapons from abroad, and future strategies like cyberterrorism. Full Story