The Central Statistics Office's agriculture section has conceded it overestimated farm income last year because it changed its methodology and data sources.
However, it said the changes would not alter the agricultural income operating surplus, estimated at 2.6 per cent for 2003.
It was accused by the IFA chief economist, Mr Con Lucey, of seriously overstating farm income for many years.
"The latest figures from the CSO show that national farm income in 2003 was €2,175 million, whereas in February they said it was €2,590 million, an overestimate of 19 per cent," said Mr Lucey.
"The difference of €415 million is equivalent to €66 a week for Ireland's [the Republic's] 120,000 farmers. It is apparent that the CSO has been underestimating costs in agriculture for many years," he said.
"When the new CSO figures are adjusted for interest on farm borrowing, they show that average farm income last year was about €16,000. This brings the CSO figures much more into line with the Teagasc national farm survey figures," he added.
"Indeed this readjustment strongly supports the case made by IFA president John Dillon during the tractorcade when the Minister for Agriculture disputed the Teagasc figures on the level of farm incomes."
Mr Lucey said the IFA would be seeking a repayment of under-compensation of VAT refunds in the past in the context of next year's pre-budget submission.
He said calculation for earlier years was based on inaccurate data; for example, the overstatement of farm income was 12 per cent in 2000 and 15 per cent in 2001, due mainly to costs being underestimated.
The CSO spokesman said it could not comment on this aspect of the matter.
He said the changes between the final and preliminary estimates were mainly due to revised cereal output and feeding stuffs data from the Department of Agriculture and Food and revised data for the potato harvest.
He said the base year for the calculation of volume indices had been changed from 1995 to 2000, but it had revised all the agricultural income figures back to 1990.