The Prison Officers' Association said yesterday the timeframe for reform proposed by the Minister for Justice, Mr McDowell, is "unacceptable and unmanageable".
National Executive Council delegates from each of the 16 branches met yesterday to discuss a document from the Irish Prison Service.
"The changes suggested by the Irish Prison Service involve radical reform to the whole Irish prison system," said assistant general-secretary Mr Stephen Delaney. "This is the first document outlining their agenda in six years.
"Mr McDowell said last week he would impose changes if there is no agreement within 90 days. We find this shotgun on the table approach not acceptable," he said.
The association did realise it was a two- way process and wanted to engage in meaningful discussions to arrive at a satisfactory agreement for all, but it viewed the demands on time as unrealistic, Mr Delaney told The Irish Times.
"We will meet the Minister head-on over the next few months," he said.
The association will meet the Irish Prison Service on Thursday to seek clarification on a number of issues.
Some of the changes outlined in the document include the introduction of an annualised hour system and the ending of the need for overtime payments.
Mr Delaney acknowledged there was a negative public perception of prison officers' overtime but said the reality was that "officers are working 60, 70, 90 hours a week to earn that money due to staff shortages".
He said: "There was in excess of 56,000 instances of staff conscripted to perform compulsory duty on their days off in the past year alone. These are issues the public are not aware of.
"There is a requirement by the State to provide a public service to the people in custody. All we want is a reasonable and fair wage for the job that we do."