The personal information of 380,000 social welfare recipients has been lost in what the Data Protection Commissioner described as a 'serious incident'.
The information was contained on a laptop stolen from the Office of the Comptroller and Auditor General in April 2007.
The data related mostly to the records of pensioners receiving benefits in 2005.
The theft was reported to gardaí and to the Department of Social and Family Affairs at the time.
5,000 of the records relate to those receiving unemployment and employment supports in 2004 from three local offices - Kilbarrack in Co Dublin, Newbridge in Co Kildare, and Cobh in Co Cork.
About a quarter of those affected were having payments made directly through their bank accounts, and the department says it is immediately contacting those people.
Over the next two weeks it will write to all others who have been affected.
The C&AG says payroll details of staff at seven public bodies were contained in two other stolen laptops.
In all 16 laptops have been stolen from the office over the last 10 years.
Minister for Social and Family Affairs Mary Hanafin has said she is very concerned on a number of grounds about the incident.
Ms Hanafin said the information included the names, addresses, dates of birth, and PPS numbers of people, adding that bank details were contained in 100,000 of the files.
She said there was no indication that the information has been used in any way.
However, Minister Hanafin said she was concerned that such information would be put on a laptop in a readable form and also that 15 months elapsed before her Department was informed last week of the details of the stolen files.