Mrs Mary Robinson today said she will step down as UN human rights chief even though many countries want her to stay.
"This will be the last year I will address the commission as High Commissioner," Mrs Robinson told delegates at the opening session of the UN Human Rights Commission in Geneva.
The Taoiseach, Mr Ahern, this evening paid tribute to Mrs Robinson saying "she was a fearless and tenacious advocate for the promotion of human rights throughout the world".
Human rights campaigners said the announcement to the UN Human Rights Commission showed that the will of the United States had prevailed against supporters of the former Irish president.
"She has paid a price for her willingness to confront publicly big governments like the United States and Russia when they violate human rights," said Mr Reed Brody of the New York-based Human Rights Watch.
Mr Brody said Mrs Robinson had incurred Washington's displeasure by criticising US treatment of Afghan prisoners held at Camp X-Ray in Cuba and by spearheading the anti-racism conference in South Africa, boycotted by the United States because of criticism of Israel.
Ms Melinda Ching of Amnesty International said, "Of course we will be disappointed. We think she was a fantastic human rights defender and will be looking to ensure that whoever takes her place will keep up the work.
Ms Ching said Mr Robinson would be "a hard act to follow."
Mrs Robinson had never publicly declared a campaign to remain in office, but she had support from Western Europe and many Arab and developing countries, diplomats said.
"Her not being reappointed is going to send a very chilling message," Mr Brody said. "If you stand up to the United States, this is what happens."
US officials said they had no immediate comment. Previously they have refused to say anything publicly on Mrs Robinson. Privately, however, US officials at UN headquarters have indicated Washington's lack of enthusiasm for Mrs Robinson.
A year ago Mrs Robinson said she was stepping down at the end of her four year term, but she agreed to Secretary-General Kofi Annan's request that she stay on for one more year.
She recently indicated she was willing to extend for three more years past the end of September, when she is scheduled to step down, diplomats said.
Mrs Robinson's disclosure came in her keynote address to the opening session of the commission, which has the United States on the sidelines for the first time in its history.
The US delegation, which failed to win re-election to the panel a year ago, is no longer sitting among the 53 member nations of the commission and has been relegated to the observer countries in the rear.
The delegation has yet to sign up to speak during the first week, which will include the annual debate on Israel's treatment of Palestinians in the occupied territories.
Instead of a large contingent from Washington, this year's delegation mainly consists of diplomats assigned year round to Geneva.
Mrs Robinson told the commission that the response to September 11th terror attacks could undermine protection of rights worldwide.
"The events of September 11 were not only of monumental consequence for the people of the United States and the victims who came from over 80 other countries," Mrs Robinson said.
"These acts were an attack against the very system of international relations on which this commission and the entire work of the United Nations is based," she added.
She said "international human rights standards are at some risk of being undermined."
Mr Robinson stopped short of singling out any countries for criticism, but she said a "sharp rise in Islamophobia, anti-Arab and anti-Semitic expression" since September 11 underscored the need to fight discrimination and racism.
"The buildings that were destroyed on September 11 can be replaced," she said. "But if the pillars of the international system are damaged or demolished, they will not be so easy to restore."
She also recalled her criticism of alleged abuses by Russian forces in Chechnya, Israeli violations in the territories and Palestinian suicide attacks against Israelis, and noted problems in Zimbabwe, Colombia, China and Indonesia.