Paramedics worked at the scene to save the officer's leg amid fears it may have to be amputated
The garda - who was on traffic duty - was attending an incident on the N25 Waterford-Cork road outside Youghal near the town bypass when he was apparently injured by a passing vehicle.
The incident occurred shortly after 7pm.
Emergency services raced to the scene including gardaí, Youghal Fire Brigade and HSE paramedics.
It is understood the garda suffered serious crush injuries to one leg as well as multiple other injuries.
Paramedics worked at the scene to save the officer's leg amid fears it may have to be amputated.
The injured officer was rushed to Cork University Hospital (CUH) where he was scheduled to undergo emergency surgery.
Gardaí have sealed off the area of road involved to allow for a full investigation by accident scene investigators.
No one else was injured in the incident, although the two other motorists at the scene were treated for shock.
Initial indications are that the garda was assisting an elderly motorist whose vehicle had broken down on the N25 between Youghal and Killeagh when he was somehow struck by a passing vehicle.
Driving conditions were said to be difficult at the time given the darkness, misting rain and large amounts of surface water.
The injured officer is based in Cork.
In a statement issued tonight, a garda spokesperson said: “Shortly before 7pm this evening an unmarked garda patrol car was assisting a broken down vehicle on the N25, at the Youghal bypass.
"The unmarked patrol car was struck by another vehicle. A Garda member has suffered serious injuries and has been removed from the scene to Cork University Hospital where he remains in a critical condition.
"The driver of the other vehicle was also removed to Cork University Hospital with non-life threatening injuries.
“The N25 at the Youghal bypass is currently closed and the scene is being preserved. Local diversions are in place.”