The incident occurred at around 2pm yesterday when four teenagers, all understood to be aged around 15, were swept off rocks on which they had been walking.
Two teenage boys managed to pull themselves ashore but the girl and another boy remained in the water.
The four teenagers were part of a 14-strong scout group from the South Circular Road branch on an organised visit to the area.
The Coast Guard in Dublin said it received three 999 calls in quick succession from people at the scene at Hook Head, with the first logged at 2.05pm.
The 117 Rescue Helicopter was despatched and was first at the scene, arriving at 2.12pm.
The girl and the boy were then both winched onto the helicopter. It is understood that while the boy was conscious, the girl was unconscious when she was recovered from the water and required CPR.
The helicopter then went straight to University Hospital Waterford, where the girl was said to be in a critical condition last night.
The two boys who managed to come ashore by themselves are understood to have suffered little more than cuts and bruising, but both the girl and boy plucked from the sea had suffered hypothermia in seas with a temperature as low as 8 degrees Celsius.
The RNLI life boat from Dunmore East and a unit of the Coast Guard from Fethard-on-Sea were also at the scene of the rescue, while a unit from the Coast Guard at Dunmore was involved in preparing the landing site.
It is understood the boy and girl were close to each other when they were winched from the sea into the helicopter and may have been holding on to one another while in the cold water.
John Lawlor, CEO Scouting Ireland, confirmed that a support team had been sent down to the area yesterday afternoon to support the other members of the group. Some of the teenagers were expected to return to Dublin in the early hours of today.
Locals said that while the winds had died down the seas in the area were still rough yesterday afternoon.