A 30-year-old Irish man has pleaded guilty in New York to narcotics conspiracy over the now-defunct "dark web" marketplace Silk Road, just months after being extradited to the United States.

US prosecutors say Gary Davis, who went by the alias "Libertas," was a Silk Road administrator in 2013 and was paid a weekly salary to carry out duties that included resolving disputes between drug dealers and buyers on the site.

The Wicklow man was arrested in Ireland in January 2014 and extradited to the United States in July, where he had faced a range of charges.

He has pleaded guilty in a Manhattan federal court to one count of conspiracy to distribute narcotics, which carries a maximum sentence of 20 years in prison, after cutting a plea deal.

He is expected to be sentenced on 17 January, 2019.

"As he admitted today, Gary Davis served as an administrator who helped run the Silk Road marketplace," said Manhattan US Attorney Geoffrey Berman.

"The purported anonymity of the dark web is not a protective shield from prosecution," he added.

Until the FBI shut it down in October 2013, the US government called Silk Road "the most sophisticated and extensive criminal marketplace on the Internet," used by vendors in more than ten countries in North America and Europe.

Texan mastermind Ross Ulbricht was convicted and sentenced to life in prison in 2015 for running the online enterprise that sold $200 million in drugs worldwide.

Operating under the alias "Dread Pirate Roberts," his trial was considered a landmark case in the murky world of online crime and government surveillance.