The State's income tax take for the first four months of the year is 3% lower than expected.
The latest Exchequer Returns show tax receipts of €14.1bn for January to April - around a third of a billion euro lower than planned.
Income tax has a shortfall of nearly €200m, while corporation tax has also come in much lower than planned so far.
The shortfall comes despite the CSO reporting earlier that the unemployment rate has fallen yet again, to 6.2%.
In all, the State ran a deficit of €2.5bn between January and April, but the Department of Finance says the underlying position is better than last year.
Peter Vale, tax partner at Grant Thornton, said: "The exchequer figures for April continue the recent trend of slightly disappointing numbers.
"Income tax receipts continue to remain relatively flat compared with the prior year, despite strong economic growth and robust employment figures.
"In contrast with the stagnant income tax numbers, VAT receipts for the year to date are strong, up 14.5% compared with 2016.
"Corporation tax receipts continue to be a cause of some concern, with the figures significantly behind both forecast figures and the 2016 equivalent, with no obvious explanation for the shortfall," he said.