DUBLIN—The United States has asked Ireland to hand over a 28-year-old man who the Federal Bureau of Investigation says is the world’s largest facilitator of child pornography, the Irish High Court heard on Thursday.

The request from U.S. Attorney General Eric Holder asks Ireland to extradite Eric Marques to face four charges of advertising and distributing child pornography, said barrister Pat McGrath, representing the Irish state.

An FBI agent, during an earlier hearing, described Marques as “the largest facilitator of child pornography on the planet.”

He is accused of using encryption technology to offer an anonymous hosting service for websites with a total of more than 1 million pornographic pictures, including images of rape and torture of pre-pubescent children, the agent said.

Marques has not made a plea in court, but his legal team has indicated they plan to challenge the extradition. His father was quoted by the Irish Independent newspaper as saying his son was innocent.

Marques, who holds both Irish and U.S. citizenship, grew up in New York until the age of 5 with his Brazilian father and Irish mother, and has lived in Ireland since then.

Marques told the court he had earned substantial income from a web hosting business that he operates in an apartment on a Georgian square in central Dublin.

His initial request for bail was refused by the judge who described him as a flight risk, citing the transfer of considerable sums of money to Romania and inquiries made from Marques’ computer about applying for a Russian visa.

The charges each carry a maximum prison sentence of at least 20 years. The FBI agent said incarceration for life without parole was a possibility.