Developer Michael O'Flynn claims three former trusted senior employees - including Patrick Cox, son of former MEP Pat Cox - all stole confidential and critical information belonging to the O'Flynn group.
He claimed the manipulation of a young PA by former investment director Patrick Cox, in what he said was the theft of nearly 37,000 documents, was shocking and something Mr Cox junior should be "downrightly ashamed of".
Mr O'Flynn, chairman and chief executive of the group, was giving evidence in a High Court case in which a number of O'Flynn companies are suing former employees Mr Cox, Liam Foley, Eoghan Kearney and four companies.
It is claimed the defendants made more than €12.5m in profit on a student accommodation project on Gardiner Street, Dublin at his business's expense using this information. The defendants deny the claims.
Mr O'Flynn said the conduct of Mr Cox was "the worst I have ever come across". Mr Cox had sought "bonus payments and bonus payments" and the group was happy to accommodate him, he said.
"But the money he got was used to set up against us and that was staggering," said Mr O'Flynn.
Mr Cox then started making false allegations about O'Flynn's keeping information from Nama, which had taken over the group's €1.8bn in loans, about projects in Coventry and Birmingham in England.
This, he said, was an attempt at blackmail and to dissuade the company from bringing court action against him and his co-defendants. It was an attempt to damage his reputation, which he had spent a long time earning.
"I don't intend losing it to a bunch like this," he said.
Mr Cox could have gone to Nama "if there was anything untoward" in the last three years but had not done so, he said.
But Mr O'Flynn got his solicitor to write to Nama summarising those allegations which he stressed were completely false.
Mr O'Flynn said his differences with Nama about policy were well known over the years but he had always operated "straight down the line" when dealing with Nama.
This time was "the toughest eight years in business and we had to fight to survive" by drawing on the company's experience, Mr O'Flynn said. "But we did not act unlawfully or deprive Nama of any money."
The case continues.