One in four pensioners faces paying more tax this year following a blunder between Revenue and the Department of Social Protection.
Some 115,000 people have been informed they could be forced to pay up to €8,000 extra for failing to declare they were in receipt of a private pension on top of their state pension.
Tax Commissioner Declan Rigney has refused to rule out back taxing those liable for the past missed payments.
“This is not an amnesty, let me be clear on that,” said Mr Rigney.
He said Revenue was analysing data it received from the Department of Social Protection which allowed it to match those who have been receiving both a state and private pension, or an additional source of income.
“We need to do the analysis, we need time to do that,” Mr Rigney told RTÉ.
“My point is, right now, it’s about sorting it, going forward as best we can.”
Individuals earning less than €18,000 a year and a couple earning less than €36,000 are exempt from paying tax on their DSP state pension.
Of the 115,000 who failed to inform Revenue of their additional income, many have claimed they thought the organisation had factored in their private pension and that they were already paying for it.
Mr Rigney advised that, in many cases, liability will be modest.
“The variety involved in this is immense. It may be that somebody has a very modest increase in their tax. It may be something as small as €1 a week,” he said.
“Then there is the top end and there has been a lot of emphasis put on the top end, the €8,000 a year if you’re married, the €4,000 if you’re single.”
However, Revenue has estimated that around 2,500 of the 115,000 who did not declare have non-state pension income in excess of €50,000.
Meanwhile, groups representing pensioners have reacted angrily to the revelation that thousands of already hard-pressed people will have to make these additional payments.
The Senior Citizens Parliament accused the Revenue in not being proactive in informing pensioners that they need to declare if they receive more than just their state pension.
Age Action defended those who had not made their payments, also claiming they were either unaware they had to or believed they were already doing so.
Spokesman Eamon Timmins said much anger and upset had been caused among the elderly community.
“The bottom line is that law-abiding citizens are now left with tax bills at the start of a year when there have never been as many demands on their pensions, and new taxes and charges to pay,” said Mr Timmins.
“It will cause further hardship for many older people.”