Prison officers at two of the country's largest prisons are taking industrial action in a dispute over staffing levels.
Officers at Cloverhill Prison stopped work for one hour this afternoon and will do the same tomorrow, while one-hour stoppages are due to take place for two days next week at Mountjoy Prison.
Pickets were placed on Cloverhill and inmates remained locked in their cells for an extra hour.
The Prison Officers' Association has accused the Prison Service of breaching local agreements on staffing levels.
It says that management expects them to continue to provide full services with insufficient staff.
The association also claims staffing levels are often "dangerously low" at the prisons and that the Prison Service is in breach of health and safety agreements.
The Prison Service has said it is disappointed by the decision, which it claims is a breach of the Haddington Road agreement.
It added that the action is occurring after extensive engagements at the Labour Relations Commission, which it said "resulted in the acceptance by both parties of a conciliation agreement drafted by the Commission late yesterday evening."