Next week, Joe Biden and Bill Clinton will arrive in Northern Ireland to join commemorations of the 25th anniversary of the Good Friday agreement. Clinton is now celebrated as one of the key players behind the agreement, but he didn’t do it alone. It took years of background efforts – of secret meetings, discreet lobbying and high-risk shadow diplomacy, by people whose names we’ve never known – to convince the United States to get involved. This week, Jonathan Freedland speaks to one of those people, Niall O’Dowd, who tells the extraordinary story of how he built a secret channel between Clinton and the Irish republican movement