An armed cordon was placed around Limerick Prison last night after five inmates staged a protest on the roof.
They began their protest shortly after 6.30pm yesterday, and a security operation was immediately put into place.
The Garda helicopter was deployed, and extra prison officers were called in to back up gardaí and armed detectives.
Gardaí could not confirm the exact motive for last night's demonstration, but it may have been caused by a decision to move a Limerick prisoner, serving a life sentence for murder, to Cork Prison.
There is also speculation that the protest was staged after a serious assault on a Limerick prisoner in his cell earlier this week.
On Wednesday morning a prisoner had an artery in his groin severed after he was assaulted in his cell by an inmate armed with a pointed object.
The victim was taken to Limerick Regional Hospital where he underwent surgery.
It is believed he has not lodged a formal complaint with gardaí. However, the attack is still under investigation.
A Garda source said that those involved in last night's protest were young men.
The area surrounding the prison, which is located in Mulgrave Street, was closed to traffic. Some gardaí took up positions in the grounds of the adjoining psychiatric hospital, St Joseph's.
A number of gardaí were also deployed to control a crowd of up to 50 spectators who were taunting the protesters.
A spokesman for the Irish Prison Service said there were five men on the roof of the prison's A wing.
The authorities were still trying to ascertain how they made their way on to the roof, he said, and added that he was not in a position to confirm the motive for the protest.
"I don't know how they got up on to the roof; we are still trying to ascertain this.
"There is a lot of speculation surrounding the motive, but we are not in a position to confirm the motive at this stage," he added.