Some 115,000 pensioners face paying more tax this year, the Revenue Commissioners has revealed.
The development comes following an exchange of information between the organisation and the Department of Social Protection.
It has matched people who receive a private pension with details of individuals claiming the State pension.
People who receive both are obliged to declare them to the Revenue Commissioners.
It has emerged 115,000 will now have to pay more tax this year.
This is because their State pensions was "never declared, under-reported or their personal circumstances have changed," the Revenue Commissioners said.
The Revenue Commissioners said in a statement: "In cases where the pension is not on record at all and their other income already brings them into the 41% tax bracket, the additional liability could reach €4,400 if single and €8,800 for a couple."
It said because the impact for taxpayers may vary depending on their particular circumstances this is explained in a range of letters that are being issued by Revenue this week.
There is no issue for 325,000 pension recipients as they have no income apart from the State pension or are exempt from tax altogether.
Income tax legislation says people receiving €18,000 or less if single and €36,000 or less for a married couple are exempt from tax.
Age Action Ireland said the letter sent to pensioners by the Revenue Commissioners has caused anger amongst its recipients.
It said law-abiding citizens are now left with tax bills that will cause further hardship for many older people.