Christopher Grant (33) struck the prison officer after he was told to return to his cell because he had no shoes on.
The case against him was adjourned for the production of a victim impact statement.
Grant, with an address at Barrygarron, Kilmeaden, Co Waterford, pleaded guilty to assaulting the man, causing him harm at Cloverhill Prison in west Dublin.
Blanchardstown District Court heard the incident happened on June 7 last year.
A garda told Judge Gerard Jones that the Director of Public Prosecutions consented to the case being dealt with summarily in the district court subject to the issue of jurisdiction being considered.
Outlining the prosecution’s case, a garda said that Grant was in prison in Cloverhill when an officer opened the cell door and asked the accused to come out so he could clean the cell.
When Grant came out, he had no runners on and his trousers were pulled up which was against policy, the garda said.
When he was directed to return to his cell, the accused headbutted the officer, who stumbled back.
Grant was restrained by other prison guards and returned to his cell.
The victim was treated for his injuries and the court heard he suffered a fractured nose.
He attended hospital and underwent one operation.
The officer had to spend some time off work as a result.
The injury had an effect on the officer’s breathing and he was now due to have a second operation on his nose.
The victim still had issues with his breathing but he had gone back to work, the court heard.
After enquiring about the victim’s progress, Judge Jones accepted jurisdiction, allowing the case to remain in the district court.
Grant’s lawyer said he would be pleading guilty.
The court heard that the prison officer was “amenable” to making a victim impact statement.
This was required before sentencing and the judge noted the plea and adjourned the case to Cloverhill District Court.
Grant did not apply for bail and he was remanded in custody.