A man in his 20s has been arrested in connection with an alleged assault at the Midlands Prison in Portlaoise.
Gardaí said they were investigating an assault causing harm at the prison on 6 December.
The man is being detained under section 4 of the Criminal Justice Act 1984 at a garda station in the Eastern Region.
It is understood the arrest is related to ongoing gardaí enquiries trying to establish a motive for the arson attack in Edenderry on 6 December that killed four-year-old Tadgh Farrell and his grand-aunt Mary Holt.