According to a BBC report, Breen, who was northern editor of the recently-closed Sunday Tribune newspaper in Ireland, began defamation proceedings against the NUJ in relation to comments published about her in the union's magazine, the Journalist in 2009.
The comments related to Breen's legal battle to keep sources confidential in her work on the Real IRA.
The NUJ issued a retraction and public apology to Breen in October 2009.
At the time, the union said: "We unreservedly accept that the allegations in the headline and article were totally and absolutely without foundation, and should never have been published by us in the first place".
Speaking to the BBC outside the High Court yesterday, Breen said she was "very pleased" with the outcome. Her lawyer, Paul Tweed of Johnsons Solicitors, added that she had been reluctant to take action.
"However, such was the gravity and outrageous nature of the offending allegations, which not only sought to undermine her professional integrity, but also could have prejudiced her personal security and that of her young family, that she was left with no choice but to issue these defamation proceedings," he said outside court.
In a statement, the National Union of Journalists confirmed that an agreement had been reached that it would pay "a substantial sum" to Breen in a confidential settlement of her libel action.
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