Killer Graham Dwyer has been injured in an attack in prison after a convicted rapist threw a chair at him in a row over a poker game.
Dwyer (42) - who is serving life in the Midlands Prison for the murder of vulnerable Dublin woman Elaine O'Hara (36) - received injuries to his arm when he put it up to protect himself as the chair came towards him.
It had been thrown by serial rapist Salman Dar (34), who is serving a sentence for raping five women.
He is considered one of the most dangerous sex offenders in the country.
The row happened at 6pm on Monday evening in the recreation room of the E3 landing in the jail.
Dwyer was playing poker with three other inmates when it is said Dar came in and asked to join the game.
Sources say Dwyer said no to the request and Dar flew into a rage and picked up the chair and threw it at the group Dwyer was in.
"He batted the chair away with his arm, but did not sustain any serious injuries," the source said.
"He required no medical attention at all and calm was restored shortly afterwards," they added.
Dar was removed from the room and the matter is being investigated by prison bosses.
The Pakistani national arrived in Ireland in 2003 with hopes of working in the IT industry. But he attacked a number of women in a series of sickening rapes, and was jailed for 15 years in 2005 for three such attacks on women he targeted between February and June in 2004.
Last year he received a further seven-year sentence, with two suspended, after confessing to two more attacks.
He is not due for release until 2018 at the earliest.
The attack on Dwyer is the first since he began his sentence in March. But he did face threats while on remand in Cloverhill due to the notoriety of his crime.
Dwyer was found guilty of luring Elaine to a remote area of the Dublin mountains before stabbing her to death for his own sexual gratification.
He had engaged in a secret BDSM relationship with the vulnerable woman and killed her just hours after she had been in hospital for her mental wellbeing.
Dwyer pleaded his innocence but was found guilty by jury.