Baby Kerrie Conway, who was born along with three sisters and two brothers on May 22, died Monday night in the hospital.
Ursula, Austin, Shannon, Karla, Eoghan and Kerrie were born via Caesarean section delivery at Belfast's Royal Victoria Hospital in Ireland with the help of 30 staff members. They weighed between 2 pounds, 2 ounces and 11 pound, 7 ounces.
The babies were born 14 weeks prematurely and were being treated in intensive care.
The birth of the sextuplets was the first of its kind in the United Kingdom in more 25 years. All six babies had been conceived naturally.
Parents Nuala and Austin Conway of Dunamore in County Tyrone, had been warned of the dangers of the multiple births, but ignored the advice of doctors.
"We knew without discussion what we both wanted," Nuala Conway said in an interview last month. "These babies are a wonderful gift from God."
The condition of the remaining five babies is unknown.