A lawyer in New York claims to know of at least 50 Irish men who may have been working at the World Trade Centre.
So far a Cork woman, Ruth McCourt, and her four-year-old daughter Juliana as well as priest Fr Michael Judge, are the only confirmed Irish fatalities.
But a spokesman for the Department of Foreign Affairs said the list of Irish casualties will grow in the coming days and weeks.
Lawyer Denis Guerin, originally from Killarney, County Kerry, says many Irish men were working for the New York and New Jersey Port Authority, which maintained the towers.
Mr Guerin says others were working for two construction companies renovating the upper storeys of the buildings. His company deals with a lot of people from Ireland wanting to work in New York and his office draws up contracts for them.
Mr Guerin believes people working in the basement of the building would have had time to get out, but he knows of others on the 106th storey who would have been cut off from all escape routes as the planes crashed into the floors below them.
He said many construction workers are unaccounted for and he had been taking calls all day about who was in the building.
"I deal with all the contracts and I'd know all these people. I can't tell you names but I know people who are missing," he said.
Ireland's Department of Foreign Affairs is taking on extra staff at its consulate in New York to deal with the grim task of identifying victims. Its Dublin headquarters now has trauma counsellors, as families call to see if a loved one is among those missing.
A spokesman is appealling to anyone who has called them over the past two days to contact them again if they had been able to track down anyone missing, so they can be eliminated from their inquiries.