An ecumenical memorial service will be held in Dublin's Pro-Cathedral tomorrow at 10.45am.
The service will be interrupted at 11am for three minutes silence.
Members of all faiths are welcome to attend in sympathy with the victims of the World Trade Centre disaster.
Cardinal Desmond Connell will lead prayers at the service while Archbishop Walton Empey will deliver the homily.
"President Mary McAleese and Taoiseach Bertie Ahern will both attend," said spokesman Fr Damien McNiece.
Up to 1,200 people can be accommodated in the Pro-Cathedral.
In response to the Government’s declaration of a National Day of Mourning, Cardinal Connell is requesting that Masses or prayer services be organised in each of the 200 parishes of the diocese to coincide, if possible, with the Ecumenical Service (starting at 10.45 a.m).
Meanwhile, a prayer and music service will take place in the Pro-Cathedral between 8pm and 11pm tonight (Thursday).
"Tonight's service is particularly geared towards young people," said Fr McNiece.
"We are conscious that many young people visit the United States every summer and many of them visit the World Trade Centre.
We planned this service particularly with young Irish people in mind who may have made friends in the United States on visits there," he added.
On Sunday, Cardinal Desmond Connell will celebrate Mass at 12.30pm.
President McAleese and Bertie Ahern are again expected to attend that Mass.