The Irish Prison Service says it is "extremely disappointed" that a planned work stoppage went ahead at Cloverhill Prison in Dublin today.
Inmates were left locked in their cells because of a dispute over staffing levels.
Two weeks ago, the Prison Officers Association served notice of planned work stoppages at two of the country's biggest prisons.
They claim staffing levels are "dangerously low" and their members cannot be expected to deliver the same full service without more officers.
Despite extensive talks at the Labour Relations Commission this week, a one-hour work stoppage went ahead at Cloverhill prison this afternoon resulting in inmates being left in their cells for an extra hour after lunch.
In a statement, the Irish Prison Service says its "extremely disappointed" with the move, particularly in light of what it claims was the "acceptance of a conciliation agreement" drafted by the LRC late yesterday evening.
Another one-hour stoppage is planned for Cloverhill prison tomorrow afternoon with similar stoppages due to take place in Mountjoy next week.