Gardai have arrested a man in his 20s in relation to an alleged assault in the Midlands Prison in Portlaoise on December 6.
He was arrested on Tuesday morning by officers from the Serious Crime Unit and is currently being held at a Garda station.
Gardai said they continue to investigate all the circumstances around the incident.
Four men in their 20s and 30s arrested on Friday in connection with the alleged attack were later released without charge and returned to the prison.
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