Galway Cathedral was filled for a special civic Mass in memory of the US victims. Hundreds more stood outside.
Over 5,000 attended the Mass at midday, one of dozens of remembrance services held in Co Galway on a day when the entire city and county came to a standstill.
Virtually every business in the city remained closed and people queued to sign a book of condolences, while others waited anxiously for news of their loved ones in New York.
The UN High Commissioner for Human Rights, Mrs Mary Robinson, joined the mayor of Galway and the corporation for the special ceremony at the cathedral.
Ecumenical services were held at St Nicholas Collegiate Church, where a book of condolences was opened, and at St John's Church of Ireland, Ballinasloe, as well as at dozens of churches throughout the west.
The most poignant prayers were for the families who had yet to hear from their loved ones.
Among those still missing are stockbroker Ms Anne Marie McHugh (35), from Tuam, who worked on the 89th floor of the World Trade Centre.
Also missing is Mr Martin Coughlan, whose wife, Catherine, is from Galway city. He had been working on the 92nd floor and telephoned Catherine at 9.10 a.m. to say that he had reached the 80th floor.
He said the fire services had reached them at that stage.