At least two prison officers have been injured during an incident at the Midlands Prison in Portlaoise.
It is understood staff intervened after a fight broke out among a group of prisoners.
One prison officer is reported to have received a knife wound.
Deputy general secretary of the Prison Officers Association Jim Mitchel said: "The information is quite scant at the moment…Two of our officers were injured and both had to be hospitalised.
"One received a blade injury and received 14 stitches…A lot of the details are sketchy at the moment."
There are reports the man was slashed across the buttock with a makeshift 'shiv' knife.