The Unlikely Informer

He is no James Bond. Nor, say the few associates still willing to talk about David Rupert, does he fit any other espionage stereotype. Yet this beefy half-Mohawk, half-German truck driver from upstate New York is said to have done what years of work by British and Irish secret agents had never accomplished: breach the wary brutality of veteran Irish terrorists and get evidence that could put their top leader behind bars.

Six weeks ago, around the time 49-year-old Rupert and his wife suddenly vanished from their Indiana home into a witness protection program, Irish police in the...