A 15-year-old Dublin girl was served at least 10 alcoholic drinks before she fell over the side of a cruise ship last January, a family friend has told a congressional hearing in Washington.
Brian Mulvaney also said the FBI has refused to help the family because , Lynsey O'Brien was not a US citizen and the drink was served outside US territory.
Mr Mulvaney was appearing on Tuesday before the subcommittee on national security, emerging threats and international relations, which was hearing evidence on cruise-ship safety.
He told the subcommittee that the cruise line, Costa Cruise, had allowed Lynsey to drink huge amounts of alcohol though the company's passenger charter said it would not serve alcohol to minors.
He called on Congress to introduce new legislation that would make US cruise companies criminally responsible for any cruise that leaves US ports.
Lynsey fell overboard on January 4th off the Mexican coast and her body has not been found. The cruise began and ended in Fort Lauderdale, Florida.
Mr Mulvaney told the subcommittee he was there to tell them about a "terrible tragedy" involving a "beautiful young girl".
He said he was a childhood friend of Lynsey's father, Paul O'Brien, and the two families went on the cruise together.
"Lynsey O'Brien, a vibrant 15- year-old girl, died as a result of a bartender on the Costa Cruise ship, Majica, serving excessive amounts of alcohol, more than 10 drinks, knowing she was a minor," he said.
Mr Mulvaney said that the bartender did this despite a "bold statement" on the passenger contract that prohibited serving drink to minors. He said there were signs posted all over the ship to that effect.
He added that any bartender working on US soil would be arrested and prosecuted for serving minors and the bar would lose its licence.
"Just because it was aboard a ship, it should not isolate the bartender and the company," he said. "I can't imagine a worse crime than plying a 15-year-old girl with so much liquor she literally died as a direct result."
He called on Congress to change the law so that US law applied to all ships that leave US ports.
Other witnesses testified about deaths and assaults on cruise ships in recent years.
However, Lawrence Kaye, a Los Angeles attorney who represents the cruise-ship industry, told the committee that the number of such cases was low and that Florida law allowed cruise ships to be sued for injuries caused to drunk customers.
He said cruise ships had been sued even though the potential harm was much less than on land, where drunk customers were more likely to drive a vehicle.
The Costa Line communications department did not reply to requests yesterday for a comment on Mr Mulvaney's evidence.