The Taoiseach has come under pressure as President of the European Council to end EU silence in the face of alleged human rights violations by the United States in the pursuit of its "war on terror" in Iraq.
In an open letter to Mr Ahern on the eve of the EU-US summit in Dromoland Castle, Co Clare, Amnesty International called on the EU to break its silence on the issue.
"With clear evidence of torture by US forces, and no limit on how high up the chain of command responsibility lies, the EU's silence has been shameful and deafening," Irish Section Director Sean Love said.
"The summit is the ideal time to confront President Bush with the strength of international opinion on the horrors of the detention centres in Iraq, Guantanamo Bay, and Afghanistan, and the other secret US locations."
The letter, signed by Sean Love and the director of Amnesty International's EU Office in Brussels, Dick Oosting, said George Bush should not leave Ireland without a clear message that the EU expects its single-most important partner to abide by the absolute ban on torture laid down in international law.
It pointed out that the EU's new constitution approved by EU leaders in Brussels only six days ago affirms the respect for human rights and fundamental freedoms as the bedrock of the European Union.
The letter said: "Surely now is the moment for the EU to back up its concern with robust calls on President Bush to ensure that the US opens the doors of its detention facilities not only in Iraq, but also in Afghanistan, Guantanamo and other undisclosed locations elsewhere to United Nations human rights experts and independent international human rights monitors."
The EU-US summit will see the largest security operation ever mounted in the Republic as anti-war protesters claimed at least 20,000 would demonstrate against President Bush's visit to Ireland.
More than 2,000 troops, four naval ships, Air Corp planes and hi-tech equipment, including decontamination units and bomb disposal kits to guard against a chemical or biological attack will surround Shannon Airport, where President Bush is due to arrive on Friday.
A massive garda operation involving more than 4,000 officers is also being mounted outside the walls of the exclusive Dromoland Castle Hotel where the president will spend the night before jetting off to attend a UN summit in Turkey on Saturday.
Police have warned demonstrators they will be held back half-a-mile and would be banned from the main road to Shannon Airport, where they will be kept outside the secure perimeter fence.