SINN Fein leader Gerry Adams was given a business-class flight costing up to €2,000 to travel from Dublin to New York for his surgical procedure.
It has emerged that Mr Adams is the only Sinn Fein member allowed to fly business class. The rest of the party's politicians are under orders to fly economy class when they are on party business.
Mr Adams was spotted in the business departure lounge in Dublin Airport by a member of the public before his trip to New York last summer.
Sinn Fein confirmed that the party's US support group, Friends of Sinn Fein USA, had paid for a business-class return flight for him.
The cost of an Aer Lingus business-class return flight generally ranges from €1,400 to €2,000. By contrast, an economy return costs around €400.
Sinn Fein said that Mr Adams uses business class flights when he is travelling abroad due to a medical condition.
Although it declined to give further details, Mr Adams publicly said last weekend that he had been in pain for five years due to his non-cancerous prostate problem. This was the reason for his successful surgical procedure in New York.
Sinn Fein has also confirmed that Mr Adams's Dail declaration did not include the first-class flight or the fact that the medical procedure in New York had been paid for by Irish American multi-millionaire Bill Flynn at an estimated cost of $30,000 (€22,400).
The party is claiming that this is in compliance with the rules for TDs on declaring gifts and travel facilities.
Over the years, Friends of Sinn Fein USA has repeatedly provided the funds to pay for Mr Adams' transatlantic flights and accommodation in luxury US hotels.
But Sinn Fein said the Louth TD was not required to declare the flights and accommodation provided to him because he was on the US trips "in the course of his functions as president of Sinn Fein".
It added: "All of these matters are accounted for in our annual report to the Justice Department in the United States."
Sinn Fein also defended Mr Adams's decision not to include Mr Flynn's payment of his US medical bill in his Dail declaration for 2012, which is due to be published shortly.
"It was a gift to Gerry for purely personal reasons and could not materially influence him in the performance of his functions as a member," said the party.
The rules governing the declaration of gifts for TDs allow them to leave out such gifts if they can show that it would have no influence on their decision-making.
Records filed with the US Department of Justice by Friends of Sinn Fein USA show that Mr Adams travelled on an Aer Lingus flight from Dublin to New York at the end of last July.
He had a medical consultation the following day.
Mr Adams underwent the "surgical procedure" in early August, was kept overnight, then spent a further nine days recovering at a friend's apartment in Manhattan.
Mr Adams had one official engagement during his time in the US. He gave a speech on investment and job creation at a dinner at the famous Bobby Van's Steakhouse in Manhattan, hosted by the restaurant's president, Joseph Smith.
He stayed at the home of Mr Smith in New York before and after the operation.