(CNN) -- Ireland's top Catholic cardinal is using the "Nuremberg defense" in the face of public outrage at his role 35 years ago in investigating one of the country's most notorious child-abusing priests, an activist said Tuesday.

Cardinal Sean Brady has been under fire over the investigation into the Rev. Brendan Smyth.

The latest scandal comes on the heels of a huge government-backed report that found the Archdiocese of Dublin and other Catholic Church authorities in Ireland covered up child abuse by priests from 1975 to 2004. Child sexual abuse was "widespread" then, the report found.

Brady's office said Tuesday that the cardinal, then a priest and teacher with a doctorate in canon law, had been asked to investigate two complaints against Smyth in 1975, but had no decision-making power. He reported his findings to Bishop Francis McKiernan, his office said, and McKiernan recommended Smyth get psychiatric help.

But John Kelly, the founder of Irish Survivors of Child Abuse, said Brady should not have remained silent about what he learned in the course of investigating Smyth, who was later convicted of dozens of counts of child abuse in both Northern Ireland and the Irish Republic. He died in prison.

"He's basically using the Nuremberg defense, he was carrying out orders," Kelly said, in reference to the justification many Nazis used in their war crimes trials after World War II.

There has been particular outrage over the revelation that two boys who filed complaints against Smyth were asked to sign confidentiality statements as part of Brady's investigation.

The oaths of secrecy were "to avoid potential collusion" between the two boys as church officials investigated the case, Ireland's Catholic Communications Office said Tuesday.

Despite his criticism of Brady, Kelly said it would not necessarily do any good for the cardinal to resign.

"He's lost all moral authority to lead, but by replacing him, it won't resolve the problem," he said, arguing that the Vatican would "just replace guys with other guys."

The best solution, he said, would be for the Roman Catholic Church to let secular authorities deal with accusations of abuse, rather than trying to handle them itself.

"They have to accept secular authority, and they can get on with the business of religion," Kelly said. "It would be in the church's own interest. Resignations in themselves aren't the answer."

Four Irish bishops tendered their resignations in the wake of the government-backed report, and Pope Benedict XVI is expected to make a formal statement, or pastoral letter, on the Irish abuse issue by Easter.

The pope himself has been under fire since it was revealed that a priest suspected of abusing children was allowed to move into his diocese when he was Cardinal Joseph Ratzinger in Germany in 1980.

The Archdiocese of Munich and Freising said in a statement Monday it must have been clear at the time that the priest, whom multiple sources identified to CNN as Peter Hullermann, was coming to get therapy for allegedly molesting children. He was convicted of abuse in 1986, after Ratzinger moved to Rome. Hullermann was suspended on Monday.

Hundreds of allegations of sexual or physical abuse of children by Catholic clergy have come to light in Germany, Austria and the Netherlands this year.

CNN's Frederik Pleitgen in Berlin, Germany, contributed to this report.