An inmate broke a prison officer's tooth when he lashed out with his head and struck him in the face during a dinner-time brawl.
Stephen Kelly (22) also elbowed a second prison officer in the face as he was being restrained during the disturbance in Wheatfield Prison, where he is serving a sentence for armed robbery.
Judge David McHugh gave him another four-month sentence, consecutive to the end of his current seven-and-a-half-year jail term.
He said the assaults were an "outrage".
Kelly, of Windmill Park, Crumlin, pleaded guilty to assaulting two officers in the incident.
Blanchardstown District Court heard prisoners were in the serving area on the ground floor at Wheatfield Prison last November 19.
Kelly was among a group of inmates getting meals at 4.30pm when a disturbance broke out between another prisoner and officers. The court heard this escalated to involve other inmates in the area and the accused became "mouthy", trying to egg others on.
He could be seen on security camera raising his tray.
Officers restrained him and he swung an elbow backwards, striking one of them in the face.
This caused a minor injury to the officer, but Kelly then lashed back with his head, hitting a second officer in the face.
This prison guard suffered a broken tooth and an injury to his jaw.
The court heard Kelly had 41 previous convictions, including three for assault.
Defence solicitor Simon Fleming said the accused had been "essentially struggling, trying to get free of the officer, which he shouldn't have done".
He said the assaults were more a case of "recklessness" by the accused.
Kelly was forthright with the investigating gardai and made admissions.
Judge McHugh said the more serious assault seemed to have caused "significant damage" to the officer.
Kelly's release date was in 2020. He did not have a bad relationship with the prison authorities and "accepts his lot", Mr Fleming said.