He was assaulted while investigating a report that a man had jumped into the River Liffey.
A crowd had gathered but when the young officer asked people to clear the area the man turned and butted him in the face.
A man was arrested at the scene and is due to appear in court early next month.
Justice Minister Michael McDowell condemned the attack yesterday.
He said he is close to pushing his Criminal Justice Bill, which includes an offence of assaulting emergency staff, through the Oireachtas before the summer recess next month. Mr McDowell also called on witnesses to help Garda investigations.
He added: "It is the duty of every citizen in the Republic, not just to prattle on about his or her rights, but to help gardai bring to justice the perpetrators of such cowardly acts of hatred against authority, those ordinary emergency workers who are just doing their job."
The attack comes after another firefighter was hit in the head with a bottle on Friday as he attended the scene of a bonfire in Grange, Douglas, Co Cork.
And earlier this month, Jonathan Forbes, 24, needed 35 stitches when a bottle was thrown through the open window of his fire engine in Dublin - smashing him in the face.
He has been off work since the attack almost three weeks ago and has been left with horrific scars.
Written by Daily Mirror
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