The ruling was made on Tuesday morning.
Former Anglo Irish Bank chairman Sean FitzPatrick has been acquitted of all charges in his trial at the High Court.
The longest running criminal trial in the history of the state has ended after more than 120 days.
FitzPatrick had pleaded not guilty to misleading the bank's auditors, and also furnishing false information about multi-million euro loans to him and to people connected with him between 2002 and 2007.
RTÉ's Orla O'Donnell broke the news.
https://twitter.com/Orlaodo/status/866966316204388352
Judge John Aylmer ruled on Tuesday morning that the investigation into FitzPatrick, by the Office of the Director of Corporate Enforcement (ODCE), did not meet the criteria of an impartial or unbiased investigation that the accused is entitled to.
He will direct the jury to acquit FitzPatrick as he says the ODCE did not seek out evidence of innocence as well as guilt.
The judge claimed that the ODCE tried to build a case against FitzPatrick, that key witnesses were coached and that material shredded during the former chairman's first trial could - potentially - have been beneficial to the defence.
Outside the court, FitzPatrick said it was "a long, tiring and difficult time" for his and his family. He thanked his legal team and said it was "a wonderful day".
