Two young men from the North were seriously ill in hospital last night after separate accidents in Cos Sligo and Donegal over the weekend.
A 23-year-old woman from Co Tyrone was also killed in a motorcycle accident in Co Donegal on Saturday.
The combination of the good weather and the annual exodus out of the North for the Twelfth weekend resulted in record crowds descending on resort towns throughout the north-west.
Mr John Greene, the regional manger for North West Tourism, said he believed accommodation throughout Sligo and Donegal had been completely booked up.
"As far as we can ascertain virtually everything was gone. I heard one man complaining that he answered the door up to 20 times telling people he was full," Mr Greene said.
He said he believed the Twelfth weekend was a lot busier than last year and that a lot of people from south of the Border had also taken advantage of the good weather to head to the beach.
One of the young men, believed to be from Belfast, was injured in a jet-ski accident at Mullaghmore, Co Sligo, at about 10 p.m. on Sunday night. He was pulling another youth on water-skis when his jetski collided with a boat.
Locals brought the man ashore and he was taken to Sligo General Hospital where he is believed to be on a life support machine. A hospital spokesman described his condition as critical.
Meanwhile, gardai in Co Donegal have now located a hit-and-run driver involved in an accident at Atlantic Drive, Bundoran, at 3 a.m. on Sunday morning in which two teenagers were injured.
A youth from Sion Mills, Co Tyrone, was taken to Beaumont Hospital, Dublin, after the accident, while a girl accompanying him from Sligo town is in a comfortable condition in Sligo General Hospital.
Gardai said after the accident the vehicle was driven at high speed towards the Border, but the driver had been identified and agreed to co-operate with their inquiries.
The woman who was killed in a motorcycle accident at Inver in south Donegal on Saturday evening was Ms Siobhan O'Neill (23), from Coalisland, who was a member of the Co Tyrone camogie team.