Farmers have warned rangers and conservationists to keep off their land.
Staff from the National Parks and Wildlife Service (NPWS) and Birdwatch Ireland are being told by the Irish Farmers' Association (IFA) that they are banned from accessing members' working farmland.
Jer Bergin, national chairman, said it was a clear message to the next government over broken promises on compensation for lands designated special areas of conservation for wildlife including hen harriers.
The IFA is also angered over restrictions on farming activities and limits on work to alleviate floods.
"When designations were first imposed, commitments were given that compensation would be paid where farmers suffered losses. However, in recent years these commitments have not been honoured," he said.
The trespass threat was included in a new poster campaign launched by the IFA in Shannon Harbour, Co Offaly.
Birdwatch Ireland, which regularly surveys wildlife in hedgerows, farmland, bogs and in environmentally sensitive areas , said it was extremely surprised.
"BirdWatch Ireland firmly believes that farmers are the custodians of some of Ireland's most important habitats and iconic wildlife," spokesman Niall Hatch said.
"We actively work to advocate financial support for farmers who are in a position to help wildlife, and we firmly believe that much more needs to be done by the State and the Irish Government to benefit and support those farmers whose actions serve to protect and conserve wildlife and biodiversity."
BirdWatch Ireland said it is fortunate to enjoy a very good working relationship with farmers and landowners.
The IFA claimed the NPWS refused to allow remedial work on many rivers and turloughs which compounded flooding problems this winter.
"Dredging and clearing of rivers will have to take place to alleviate the problems associated with flooding. NPWS cannot be allowed to stand in the way of this," said Tom Turley, the group's SAC project team chairman.
A Government spokesman said the Department of Arts, Heritage and Gaeltacht and the Department of Agriculture work closely with the IFA on a range of issues, including land management in designated areas in order to protect heritage.
"It is inaccurate to suggest that the NPWS has obstructed flood alleviation works," a department spokesman said.
"The department is strongly committed to working with the IFA and other farming organisations to the benefit of those living and working in rural Ireland."
Farmers can apply for compensation if land is designated SAC, a Natural Heritage Area or a Special Protection Area to protect plants or wildlife, breeding grounds or globally important sites.
Payments are based on how productive the land could be and decisions can be appealed to independent arbitration.