Knud Wollenberger was a mathematician, poet and TV editor, an East German who emigrated to Ireland. But he was also a man with a dark past. He was born in Copenhagen in 1952, the son of a German émigré returned from the US and his Danish wife. The family moved to East Germany in 1955, at a time when the Communist state had eased up a little after the death of Stalin in 1953, and Wollenberger was able to benefit from an élite education.

After a visit to West Berlin, where he met an American, he was interviewed by Stasi officers on his return to East Berlin. They persuaded him, in 1972, that it was his duty to become an Inoffizieller Mitarbeiter (IM), an unofficial collaborator of the Stasi. He later claimed that he had little choice, but admitted that this gave him a feeling that he was part of a great enterprise. He was known to his case officer as "Donald". He had joined a growing army of IMs – by 1985 there was one for every 120 citizens in East Germany.

On the orders of his superiors, "Donald" dated and married Vera Lengsfeld, and reported on her from 1982. She was born in East Germany in 1952, the daughter of a Stasi officer. After studying philosophy in Leipzig and Berlin, she worked as a lecturer and researcher at the Academy of Sciences and became a member of the ruling SED party. In1981 she left the academy and worked as an editor. She became a born-again Christian and was active in peace and human rights groups. She co-founded the Pankow Peace Circle in 1981, and the Church from Below in 1986. Due to take part in protests against the stationing of Soviet nuclear missiles in East Germany, she was expelled from the SED in 1983 and sacked from her job.

In January 1988 Lengsfeld was arrested in advance of the annual demonstration in honour of the revolutionaries Karl Liebknecht and Rosa Luxemburg in East Berlin. She was carrying a poster declaring: "Every citizen has the right to express his opinion freely and openly". She was tried for "attempted riotous assembly" and although given a custodial sentence was allowed the option of leaving East Germany on a temporary visa – effectively a way of deporting her. In February 1988 she went to Cambridge, where she studied philosophy of religion at St John's College, gaining an MA. On 9 November 1989, just before the Berlin Wall was opened, she returned to East Germany, joining the Greens. She won a seat in the democratically elected parliament in March 1990, and to the united Bundestag in October 1990. She later crossed to the Christian Democrats and remained in the Bundestag until 2005.

With his shaggy beard, long hair, gaunt features, jeans and thick woolly pullover, Wollenberger fitted in well with the 1980s dissidents. However, throughout the period he was delivering reports on Lengsfeld, her son from a previous marriage, and her colleagues. On the surface he was a supportive, loving partner and devoted father and stepfather. In reality, he divulged even the most intimate details of his marriage.

Having discovered the truth from the Stasi archives, Lengsfeld divorced Knud in 1992. Wollenberger did not face prosecution and did a variety of jobs in the arts and media. For a time he worked in radio in Dortmund and he was later a TV editor in Berlin. In 2003 a collection of his poetry was published under the title Azurazur. He moved together with his partner, Christiane, to Scariff in County Clare in 2009. They married there in 2010. By then he was suffering from muscular dystrophy, and was confined to a wheelchair. He was visited by his son Jonas shortly before his death. Remarkably, after his pleas and protestations of regret, Lengsfeld said that she forgave him.