The Department of Social Protection paid €28.7m to welfare recipients who had significant amounts of money invested in financial institutions.
An investigation by the department has resulted in €11.7m being paid back, according to documents seen by RTÉ News.
The probe focused on Deposit Interest Retention Tax (DIRT) information provided by Revenue.
The department then matched this data with its records and examined more than 1,000 cases.
This revealed that the high interest paid by financial institutions meant some people on Social Protection payments had "invested significant amounts of capital."
"These cases were referred for review where there was a possibility the means from these financial savings had not been disclosed," a statement from the department said.
Some of the welfare payments looked at included Jobseeker's Allowance, One Family Parent Allowance and Disability Allowance.
The first part of this investigation has now finished but a second one is planned, the department has confirmed.