Also Monday, the Vatican announced the pope had accepted the resignation of an Irish-born archbishop who had led the Benin City diocese in Nigeria and faced accusations that he carried on a 20-year relationship with a woman that began when she was 14.
Archbishop Richard Burke, 61, had been suspended. He was the latest bishop to resign amid the church abuse scandal, as Benedict moves to get rid of bishops who either admitted they molested youngsters or covered up for priests who did.
Child-abuse scandals have caused exceptional trauma in Ireland, a once-devoutly Catholic nation. An Irish government collapsed in 1994 amid arguments over its failure to extradite a pedophile priest to Northern Ireland. Since 2002, a government-organized compensation board has paid out more than euro800 million ($983 million) to 13,000 people abused in Ireland's church-run residential institutions for children.
The investigation in Ireland will deal with the handling of cases of abuse and providing assistance to victims. It will begin in four archdioceses, including Dublin, and then be extended to other dioceses, the Vatican said. It will also look at seminaries and religious houses.
The nine investigators will look at the procedures currently in place to prevent abuse and seek ways to improve them.
The pope invited "all the members of the Irish Catholic community to support this fraternal initiative" and hoped the investigation will be "an occasion of renewed fervor in the Christian life, and that it may deepen their faith and strengthen their hope," the Vatican statement said.
The investigators named by Benedict include the archbishops of Westminster in England, Boston, Toronto and Ottawa.
Timothy Dolan, the archbishop of New York, will investigate the Irish seminaries as well as the broader issue of priestly formation. Two nuns were appointed to investigate religious institutes for women.
"My love for the faith of Ireland, and my own background in priestly formation, make me grateful for this assignment, and I look forward to close cooperation with my brother bishops, priests, religious, and the faithful of Ireland," Dolan said in a statement on his official blog, "The Gospel in the Digital Age."
Cardinal Sean O'Malley of Boston will investigate the Dublin archdiocese. O'Malley was brought in to Boston after a priest sex abuse furor erupted there in 2002, prompting Cardinal Bernard Law to resign.
"The Church must be unfailingly vigilant in protecting children and young people," O'Malley said in a statement posted on the Dublin archdiocese's website. "It will also be important to respond to the concerns of the Catholic community and the survivors in the manner that will promote the process of healing."
Dublin Archbishop Diarmuid Martin praised O'Malley's appointment, saying his "experience and personal commitment render him particularly suited" for the task. Martin, who has been trying to root out abusive priests, called the investigation an "important element" in the purification and renewal of the Dublin church, which "addresses the truth of a dark moment in its history."
But U.S. victims of clerical abuse were not impressed by Benedict's selections, saying some of the bishops themselves had "troubling" records on confronting abuse.
"We must look outside a largely complicit church hierarchy for real solutions to this devastating ongoing crisis," said Barbara Dorris, outreach director for the U.S. victims' group Survivors Network for those Abused by Priests.
"For an apostolic visitation to have any chance of success, the participating bishops cannot be guilty of the same offenses they are investigating," said BishopAccountability.org, which conducts research into the abuse crisis.
The pope apologized for the chronic abuse in his unprecedented letter to Ireland, rebuking church leaders for "grave errors of judgment" and appealing to priests still harboring sins of child molestation to confess.
Three Irish bishops have stepped down since December, and there have been calls for the country's top prelate, Cardinal Sean Brady, to leave because of his handling of a notorious child rapist.
In the Burke case, the St. Patrick Missionary Society said Burke resigned for "his failure to observe his oath of celibacy."
However, the society said its local investigator in Nigeria never found evidence that Burke began having sex with the woman when she was 14. Burke said the sexual relationship began only after she turned 18, while he served as a priest in Warri, a city in the oil-rich Niger Delta.
"(Burke) has apologized to all those whom he has hurt by his actions and has taken full personal responsibility for what he has done wrong," the society said.
Burke served in Warri for about a decade before heading back to Ireland. In 1996, he returned to Warri and later became bishop, sometimes serving as a negotiator between Nigerian militants who kidnapped foreign oil workers and the oil companies.
Burke became archbishop of Benin City in 2007.
Catholicism is the fast-growing religion in Nigeria, Africa's most populous nation with 150 million people.