Christopher, from Rathmore, Co Kerry, dived into the River Blackwater, which is 4m deep in places, and failed to resurface.
His friends ran from the scene at Farrankeale, near Knocknagree, North-West Cork, to a nearby house, and some people came back with them to look for Christopher. He was pulled from the water over 25 minutes after he had disappeared.
CPR was administered and an air ambulance flew Christopher to Kerry General Hospital, Tralee. The initial plan was to take him to the Mid-West Regional Hospital in Limerick, but it was decided to take him to Tralee due to the urgency of his condition.
O’Sullivan family members were given a garda escort to Tralee so they could be with him.
However, he did not regain consciousness and was later pronounced dead.
Some of his friends also visited the hospital on Sunday night to bid farewells.
Christopher was a fifth-year student at Scoil Phobail Sliabh Luachra, Rathmore.
The school career guidance counsellor was available for students yesterday, while representatives of the national educational psychological services for schools and the state exams commission also attended.
Christopher’s passing is the third death in tragic circumstances this year in his particular school year.
Rathmore parish priest Fr Pat O’Donnell, who spent time with the family yesterday, said no words could ease the pain of his parents, Sharon and Jerry, and three brothers, Daniel, Tyrone, who is on the way home from Australia, and Tomas, who is returning from England.
“We are mindful, too, of Christopher’s friends in school who equally share in the sadness of this time,’’ said Fr O’Donnell.
Victories for Rathmore in the Kerry county football championship and of Knocknagree in a Co Cork championship were being celebrated in the area when the tragedy occurred.
“Rathmore is a place of friendly rivalry between Cork and Kerry, but yesterday evening the border disappeared and all are now united in sorrow,’’ said Fr O’Donnell.
“This area is renowned for its community spirit. In the days ahead, the green and gold and the red will be as one.’’
Rathmore GAA club described Christopher, who played with their minor team, as a fighter who had battled bravely but sadly succumbed to his injuries.
“His sudden death has left the club and the wider community in shock,’’ said club chairman Eoin Casey.
“Our thoughts and prayers are with his family during this terrible time.”
Rivers are among the most dangerous places for swimmers, the Irish Water Safety group warned yesterday.
IWS chief executive John Leech said 62% of all drownings occur in inland waters, and 37% of drownings occur in rivers.
“Rivers are very hazardous,” said Mr Leech.
“ The water can move so quickly and people can get pinned and snagged by branches, trees and other obstacles. Shallow areas can also lead suddenly to deep pools.
“People can drown much faster in fresh water than in salt water. The water temperature is also much cooler in rivers, which can result in cold shock and hypothermia.
“We always advise people to swim in lifeguarded areas and we had no incidents over the weekend of fine weather in such areas. In areas with no lifeguards, we always say to people that at least one person in a group should have lifesaving training.”
Mr Leech said people going swimming in inland waters should also consult local residents, who would be able to tell them about any dangers that existed.