The stop-start nature of the harvest to-date means that over 50pc of the winter wheat and spring barley crops are still to be cut.
Carlow-based advisor Pat Minnock said many farmers were restricted by the rains to a few hours cutting each day. Progress had slowed to a snail's pace as a consequence.
There are also fears of sprouting in winter wheat as the warm and humid weather persists. Mr Minnock said there were no firm reports of sprouting yet, but the high temperatures and moist conditions meant that it remained a serious threat. Despite the rain, temperatures are expected to hold in the 18-20°C range for the rest of the week.
Meanwhile, increased lodging is reported in spring barley, particularly around field headlands.
"The next week will be critical for this year's harvest. If we get a good few days in a row then a lot of the crop could be mopped up quickly, but getting the weather is the problem," said Mr Minnock.
The IFA estimates that close to 60pc of the spring barley crop remains to be cut in the southeast, but more progress has been made on winter wheat.
However, just a third of the spring barley and winter wheat crops have been cut in north Dublin and north Leinster to date.
With wetter conditions forecast for the northern half of the country this week, there are fears that harvesting will be severely disrupted. The IFA's Liam Dunne said the weather was exacerbating an "already precarious income situation" for tillage farmers, as yields are set to come in below last year's levels and "prices remain on the floor".
"Weather is playing havoc as a significant portion of the main winter wheat and spring barley crops remain to be cut," Mr Dunne said.
On a more positive note, demand for straw remains strong.
Although little baling has taken place over the last week, the price of 4x4 bales of wheaten and barley straw in Cork is reported to have hit €20/bale out of the field, up from €18/bale.
Meanwhile, Mr Dunne advised growers who suffered serious weather-related crop losses in 2016 that application forms for the Crop Loss Scheme had to be lodged with the Department of Agriculture by this Friday, August 25.