Formal judgment is set to be entered in their favour after liability was admitted by a law firm involved in the sales process.
The investors paid out 50% deposits for apartments in Calabria in southern Italy nearly six years ago.
However, the proposed El Caribe development never took place. Legal proceedings were launched at the High Court in Belfast against Gabriele Giambrone and his firm, Giambrone & Law.
It was alleged that the defendants breached their professional duty to protect the deposits.
The action also claimed Mr Giambrone operated under a conflict of interest, did not properly advise the investors and released their money to a third party agent without authority.
But just ahead of a planned trial, liability was conceded, according to a lawyer for the plaintiffs.
It is believed to be the first legal victory of its kind for Northern Ireland property buyers involved in overseas purchases.
A judge will now decide on issues involving currency rates before making a formal ruling for the plaintiffs on Monday.
Their lawyer, Simon Chambers of Russell & Co. Solicitors in Newtownards, said the case was fought "tooth and nail" right to the end.
He said: "The defendants have insisted on contesting these matters bitterly until at long last they admitted liability on the eve of the trial.
"Judgment will now be entered for the lost monies."
Mr Chambers, who has already settled individual claims by other investors, confirmed that around €1.2m was paid in deposits.
He vowed that his clients would now fight on to enforce the judgment and seek to recoup their lost money.
"These 13 individuals invested in 2007 in what was sold to them as a dream holiday home in the south of Italy," he said.
"The defendant, through his law firm which was established in London, represented that they were specialists in overseas property transactions and would ensure that all appropriate legal checks would be carried out and proper protection would be in place to safeguard against risks to my clients' money."
Mr Chambers added: "We are proud to obtain this unique and historic judgment against these defendants who are facing a large number of claims both in this and other jurisdictions.
"We are delighted that our clients can finally see some justice done."