The cereal crop, which was already predicted to drop by nearly half a million tonnes, has been further affected by the bad weather last week.
Last year tillage farmers produced a record-breaking crop of nearly 2.5 million tonnes of grain, but this year's yields are expected to return to the normal yearly average of two million tonnes.
Jim O'Mahony, Teagasc tillage expert, said there could be harvest losses in some of the southern counties because of the high rainfall last week.
He said there had been severe damage caused to spring barley crops because of exceptional rainfall, which had seen up to 70mm of rain in some southern areas.
He added that he expected some crop losses and that lodged (flattened crop) would need protection from birds until harvest.
Land ploughed this year was estimated by Teagasc, the agriculture and food development authority, at 265,000 hectares - a fall of 10 per cent on the area planted last year.
Teagasc experts said that the area of wheat planted was estimated to be 92,000, 10 per cent down on the previous year, and barley plantings fell by 14 per cent to 158,000 hectares.
He said the total projected tonnage is 1.989 million tonnes. This puts the crop at a similar output level to 2002/03, but more than half a million tonnes less than the record harvest of 2004.
Wheat production is estimated to be about 863,000 tonnes. Barley production is projected to exceed one million tonnes, and the total oat crop is estimated to yield just over 111,000 tonnes.
He said the winter wheat crops were generally good and harvesting was expected to get under way in the next week.