The President, Mrs McAleese, led Irish tributes to the fallen of September 11th.
As she attended a memorial lunch hosted by the Rotary Club in Enniskillen, which observed a minute's silence as a mark of respect, a statement by her praised American resilience and courage.
The passage of time had not lessened in any way the "enduring, shocking images of the cruel atrocities" which were visited on the United States, she said.
"Our American friends, among them so many members of our Irish family, were sorely tested that day.
"Today we reflect with admiration and respect on the courage, selflessness, resilience and fortitude with which they responded to that tidal wave of agony, which broke over them but did not break them.
"At the root of those dreadful events in New York, Washington and Pennsylvania was mankind's most potent weapon, hatred.
"Its causes are complex, its effects catastrophic, as we in Ireland know only too well."
In Belfast, councillors joined the US consul-general, Ms Barbara Stephenson, in a short but moving commemoration at City Hall.
Some 200 attended the ceremony in strong sunshine to dedicate an American maple tree and unveil a memorial plaque to those who died in last September's attacks.
The Sinn Féin Lord Mayor, Mr Alex Maskey, led the tribute to the dead alongside the UUP councillor, Mr Chris McGimpsey. In attendance were councillors from both sides of the chamber.
Mr Maskey recalled a visit by councillors to Ground Zero which he described as "a truly all-consuming experience".
He referred to the many stories of heroism that emerged, adding: "All of these accounts humble us, but they do also inspire us, to stand strong and resolute side by side, and to take courage for many of those who faced the darkest night have come through it."
Emphasising the international dimension of the atrocity, Ms Stephenson praised the response to the outrage from the people of Belfast.
"Grief has been leavened with gratitude to the people of Belfast," she said.
She recalled what she termed "the tremendous outpourings" of support and referred to the floral tributes which piled up at the door of the consulate, the prayers and the fund-raising for victims' families.
The Lord Mayor's chaplain, Father Tomas McGlynn, led prayers for peace.
"Do not let your hearts be hardened by those who did this," he said.
Traffic around the City Hall was halted by police while Mr Maskey called a one-minute silence. The Fire Brigade Union then laid a wreath at the foot of the maple tree "in memory of those who died needlessly" .
Afterwards Ms Stephenson told The Irish Times that the tragedy of September 11th could act as a force for reconciliation in Northern Ireland.
Earlier, a display of newspapers reflecting the events of September 11th was opened at the city's Linenhall Library.
The event coincided with the exact moment the first aircraft struck the World Trade Centre.
At Belfast International Airport, representatives of the four main churches led a commemoration service, and a minute's silence was also observed.
The First Minister, Mr David Trimble, attended a service in St Paul's Cathedral in London.
The Deputy First Minister, Mr Mark Durkan, was present at a memorial Mass at Clonard Monastery in west Belfast.