Fórsa said a recommendation would mean a return to the standard working week for State employees that existed before extra unpaid hours were introduced in July 2013.
The public service union said there would be a minimum working week of 35 hours.
The extra hours were imposed from July 1, 2013, and increased public servants' standard working week to 39 hours for those who worked between 35 and 37 hours. The working week for those who were on a 35-hour week or less rose to 37 hours.
The hours of those working 39 hours or more a week stayed the same.
The Department of Public Expenditure previously estimated that the cost of removing the hours would be €645m a year, equal to over 10,000 extra full-time workers.
It claimed the additional hours brought average weekly hours worked in line with the private sector.
It is understood that Public Expenditure Minister Michael McGrath was made aware of the recommendation last Friday and Fórsa and the Irish Nurses and Midwives Organisation (INMO) met to discuss it yesterday.
Fórsa welcomed the recommendation by the independent body, chaired by Kieran Mulvey, which was set up under the Building Momentum public service agreement.
It said the recommendation meant working time would return to "pre-austerity levels" from July 1, 2022.
Fórsa said the extra working time was introduced for thousands of mostly female public servants as an alternative to a third pay cut under the Haddington Road Agreement(HRA).
"If the Government accepts Mr Mulvey's recommendation, it will unwind the final element of the HRA's austerity measures," said Fórsa.
The union noted that temporary pay cuts for more senior public servants under the same deal have been restored in recent years. Pay restoration for the top earners will be implemented this July.
Fórsa general secretary Kevin Callinan said the recommendation would remove a longstanding and debilitating drain on morale and productivity across the civil and public service.
"The additional hours introduced under the HRA fell hardest on women with caring responsibilities, and they have been increasingly counterproductive in terms of service delivery, morale and productivity," he said.
"This recommendation allows the Government to remove the HRA's final outstanding austerity-era measure and, with it, a deep grievance among many - mostly lower-paid - public servants."
He said he was confident that the return to pre-austerity arrangements could be achieved without excessive cost or damage to public service quality or output.
"It has never been correct to assume that increased working time equals increased productivity," Mr Callinan said.
"Indeed, civil service departments temporarily reduced the working day to seven hours from 7.24 during the Covid-19 pandemic, largely to facilitate social distancing, and productivity was at an all-time high during that period."
INMO general secretary Phil Ní Sheaghdha said the recommendation will help the retention of nurses and midwives.
She said the move can be achieved without an excessive additional cost to the Exchequer or damage to output.