The Revenue and the Department of Social Protection are checking the names and addresses of those people who are claiming tax relief at source on joint mortgages.
The Irish Independent reports that they are cross-checking names against those who are claiming lone parent allowance to see if any of them are actually living together.
The State is spending €400m on mortgage interest relief this year - as well as a further €1.1bn on lone parent allowance.
Those who apply for both payments have to supply their name and address - making it relatively easy to cross check the records. The lone parent allowance amounts to €188 a week, plus €29.80 for each additional child, and is paid until a child reaches the age of 14.
Mortgage interest relief is a tax credit that can be worth up to €5,000 a year to a married couple - and is applied directly to the mortgage repayments a person makes from their bank account.
This is going to be abolished for new entrants at the end of next year.
The Revenue said results from the investigation into joint mortgages were not yet available but said that "appropriate action" would be taken where required.
It is the latest indication that the State is making more use of the benefits of "data matching" to root out social welfare fraud.
Fine Gael Limerick county TD Patrick O'Donovan obtained details last week from the department about its use of Revenue records to examine almost one million property sales, purchases, and transfers over the past three years.
By cross-checking the records against its own computer databases, it found that more than 40,000 records related to social welfare recipients.
