The public has been asked to observe a minute's silence at 1.46 p.m. today to mark the first anniversary of the attack on the World Trade Centre in New York.
The time commemorates the moment the first aircraft went into the Twin Towers.
The US embassy will observe the silence and has asked the public to join the commemoration.
The Government has sent a memo asking all Departments and offices to observe the minute's silence.
The President, Mrs McAleese, who will be in Co Fermanagh, will observe the silence while at a lunch hosted at the Rotary Club in Enniskillen.
Yesterday she said: "The passage of time has not lessened in any way the enduring, shocking images of the cruel atrocities which were visited on the US that day.
"Today we reflect with admiration and respect on the courage, selflessness, resilience and fortitude with which our American friends responded to that tidal wave of agony which broke over them but did not break them.
"Humankind owes it to those who died on September 11th and to all whose lives are blighted by hatred to redouble our efforts to promote a global environment in which festering bitterness and distrust give way to love and respect for every human being."
A number of commemorative religious services are to be held today.
The US embassy said in a statement: "The American people are deeply grateful for the support of the people of Ireland as, together, we mourn the thousands of lives lost on that terrible day."