Amy Brady, who was born with a heart defect, is a patient at the Royal Belfast Children's Hospital.
The 10-year-old girl, from Magherafelt, has already undergone three operations.
She has been on the waiting list for a transplant since January this year.
Amy, a sixth-grader at Spires Consolidated Elementary School in her hometown, will require round-the-clock monitoring by specialized doctors and nurses and medication until a suitable donor is found.
That means Amy and her family will have to stay in the hospital.
The Clarke Clinic at the Royal Infirmary has been her home for the past 24 weeks and she will remain there until she undergoes transplant surgery at the specialist Freeman Hospital in Newcastle.
Amy's father, Stephen, 36, had to quit his job as a roof insulation fitter to spend more time with her while she was in hospital, and the family now faces an unmanageable financial burden because of travel and household bills.
The couple, who also have son Taylor, 14, set up a GoFundMe page on Tuesday with a target of £3,000 – and by yesterday afternoon more than £5,000 had already been donated, well beyond the target.
The fundraiser is also supported by Cliftonville FC, with North Belfast Reds supporter Amy having previously served as mascot at Solitude matches.
Her favourite player is striker Joe Gormley, who made a surprise appearance at the Royal in May to say hello.
Amy's mother, Valerie, 36, said she never imagined the family's appeal would receive such a huge response.
“I can't believe how well it's gone and I never imagined it would have such a big response,” she explained.
“We've been amazed at how kind everyone has been and we're so grateful for all the help you've given us to help us reach our goals and even more.”
“This just shows how people can come together and support each other in times of crisis.”
“This money will be a huge help to us and take some of the pressure off of us as it's one less thing to worry about whilst we wait for Amy's operation. It's also really heartening for Amy to know how much people care.”
Anyone wishing to donate can visit the “Aimee's Wait For A New Heart” page on the GoFundMe website.
