The government of the United States refused to back down on its stance that John McManus was not a resident of Ireland when he won close to $20 million from gambling in the United States, even though the businessman held an Irish passport.

The 64-year-old investor, racehorse owner and gambler filed a complaint in September at the U.S. Court of Federal Claims, in which he claimed the U.S. government “wrongfully withheld” $5.22 million to cover the taxes of his $17.4 million gambling winnings.

McManus—one of Ireland’s richest—won the money playing “either backgammon or poker” in 2012, but the payer sent $5.22 million to the Internal Revenue Service as withholding. This prompted the businessman to file a non-resident U.S. federal income tax return, citing a 1997 double-tax treaty between the U.S. and Ireland.

Income from gambling is taxable in the United States, but McManus’s lawyers said the man was a resident of Ireland in 2012, and under the country’s provincial tax structure, whatever income he earned outside the country is not subjected to tax. Instead, McManus has to pay a flat tax of €200,000 under the domicile levy.