Prison officers have shared more than €1.16m since 2018 in an allowance for having to deal with inmates at the country's high-security jail.
Officers in Portlaoise Prison are paid an "environmental allowance" due to the stressful working conditions at the jail.
According to figures from the Irish Prison Service, 317 officers earned €208,822 - or an average of €659 each - in the first half of last year.
In 2019, 330 prison officers were paid a combined total of €480,706 - or the equivalent of €1,456 each in the environmental allowance.
Altogether, €1.16m was paid throughout 2018, 2019, and the first half of last year.
The allowance was first introduced in 1982 as part of an agreement between prison bosses and the Prison Officers' Association to compensate for the "abnormal tension and strain" of working in Portlaoise Prison.
At the time, conditions at the prison were particularly difficult with a large number of republican paramilitary prisoners imprisoned there at the height of the Troubles.
The allowance is payable to all officers at the jail, including clerical staff based at the prison.
The payment was ceased in 2009 for officers newly transferred to the prison.
However, a civil service arbitration board later ruled it should be restored for all.
For staff that joined before April 1995, the payment works out at around €1,250 annually.
For other prison officers who began service after that, the rate is slightly higher - because of higher pension contributions - and is paid at the rate of around €1,470 a year.
The Prison Officers' Association (POA) said the continued payment of the allowance was fully justified given the harsh pressures of working in Portlaoise.
The allowance goes back to the eighties and it has been subject to arbitration," said POA general secretary John Clinton.
"The type of prisoner you have there are paramilitaries or those involved in extremely serious crime, including gangland activity.
"We have nowhere else with these conditions - and that's recognised between ourselves and our employers."