SINN Fein president Gerry Adams underwent laser surgery to deal with a prostate problem in New York last summer.
The previously undisclosed visit emerged in documents filed by the party's support group in the United States.
Mr Adams underwent the "surgical procedure" in early August, was kept overnight and then spent a further nine days recovering at a friend's apartment in Manhattan.
Sinn Fein declined to give precise details of Mr Adams's prostate problem. But it confirmed that it was "not cancer" and that the treatment he received had been a success.
The party said he had attended a medical consultation while he was in New York. It said a successful procedure followed which required him to stay in hospital overnight.
"Mr Adams was advised that the procedure he required – which involved laser treatment – was best done in New York," it said in a statement.
Only men have a prostate gland. Around one in eight men in Ireland will be diagnosed with prostate cancer in their lifetime. But there are also other non-cancerous prostate problems such as benign prostatic hyperplasia, an enlargement of the prostate gland.
But it can be addressed with laser treatment.
Mr Adams had already been in the US for a number of days before the operation, arriving on July 30 ahead of a medical consultation the following day.
Details of his US trip emerged in documents filed by Friends of Sinn Fein, which raises large amounts of money in the US but is not allowed to send any back to Ireland.
Sinn Fein said that the issue of cost of the medical treatment, which is notoriously expensive in the US, was a matter for Mr Adams himself.
"Friends of Sinn Fein did not pay any costs toward this treatment nor was there any cost to the Irish taxpayer," it said in a statement.
In the 12 months to October 31, Friends of Sinn Fein raised close to $500,000 (€383,000), from individuals, companies and unions across the US.
Mr Adams had one engagement during his time in the US, 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 before and after the operation and Mr Adams talked about job creation and investment at the dinner.