The patient was due to be transported from a residential home in Carryduff to Royal Victoria Hospital on February 12, but its arrival was delayed due to higher priority calls.
NIAS said it received a GP call requesting an ambulance at 1.42pm and a response time of four hours was agreed.
A spokesperson said: “Ambulance control identified at 5.04pm that – due to the existing workload for higher priority calls – an ambulance would not be available within the agreed timescale.
“At 8.25pm an ambulance was allocated to this call but was ‘stood down’ en route to attend another call.”
Calls were received at 8.34pm and 10.14pm requesting updates on its arrival.
NIAS said an intermediate care ambulance was allocated to the call at 10.35pm, adding: “At 11.06pm the call was upgraded to a Category A call and an A&E crew also attended the scene. Regrettably the patient died at the scene.”
Shane Devlin, NIAS chief executive, said: “I regret the fact that we were unable to respond to this patient’s needs within the timeframe agreed with the GP and even longer.
“I would like, on behalf of the service, to apologise to the relatives of the patient who have a right to expect answers to any questions they may have in relation to how this call was handled and we would be happy to facilitate this.”
He said the incident has been reported as a ‘serious adverse incident’ and will be investigated accordingly.