EU nationals who receive child benefit payments for children who live abroad are now required to certify every three months that they are still employed in the State.
The measure is aimed at plugging a gap in the social welfare system that has left it vulnerable to fraud by migrants who continue to receive benefits even after they have left the country.
An investigation took place in Co Waterford last year into reports that some Polish nationals were flying in and out of the country in order to claim welfare benefits.
Social welfare officials estimate that the cost to the State of paying EU nationals child benefit and the €1,100-a-year childcare supplement for non-resident children will be up to €140 million in 2008.
However, migrant groups last night reacted angrily to the new measure, which was never formally announced by the Department of Social and Family Affairs but has been in operation since late last year.
The Migrant Rights Centre said it discriminated between Irish and EU nationals and placed an unfair burden on immigrants who were legally entitled to the benefit.
EU nationals in receipt of child benefit will receive four letters a year from the department, which they must get stamped and signed by their employers to show they are still in insurable employment.
If a person does not provide a reply to the department's letter within 21 days, the child benefit will be discontinued.
A similar regime will apply to non-Irish nationals who are receiving child benefit for children who are resident here. They will be required to certify twice a year that they and their children continue to reside here. In these cases, welfare recipients will have to provide details of the creche or school that their children attend. They, too, will have 21 days to respond to official letters.
The first batch of letters, which are being sent out on a staggered basis, were put in the post last November.
Under EU law, employees from any member state can claim child benefit in the country they are working in, even if their children are living in their home country.
The amount of money paid to children of migrant workers has proved politically controversial, with Fine Gael calling for the childcare supplement to be changed to exclude payments to children of EU migrant workers living abroad.
However, the Attorney General has advised the Cabinet that any attempt to discriminate between Irish and EU workers in the payment of child subsidies would be illegal under EU law.
Officials say it takes up to 15 months to process claims from EU nationals for child benefit. In contrast, it takes a number of days to process claims for children born in Ireland.
The Migrant Rights Centre said the move to place checks on EU welfare recipients unfairly discriminated between Irish and non-Irish welfare recipients.
Jacqueline Healy said the organisation was not aware of the move and that it would carefully assess its impact on migrants here.
However, she said that providing details to the department every three months sounded unreasonable and unnecessarily onerous.
"Many migrants are staying here for a long time and contributing tax and PRSI. They are legally entitled to these payments, so it doesn't seem fair that they should be subjected to this kind of treatment," she said.
Ms Healy also pointed out that many people who applied for child benefit had returned to their country of origin by the time their claims were processed.
"We have cases where it is taking a year-and-a-half to clear the backlog of child-benefit claims. When we have inquired about this, all we have been told is that there is a major backlog," she said.