A man in his 20s who had been arrested in connection with an alleged assault at the Midlands Prison in Portlaoise has been released without charge.
Gardaí said they were investigating an assault causing harm at the prison on 6 December.
The man has returned to Midlands Prison.
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.