The Northern Ireland Ambulance Service (NIAS) is beginning a review today of its operational activity after service across Northern Ireland operated at 70% of its normal level on Saturday night.
Thirty-seven crews covered the whole of the region compared to the usual 52.
That meant that every one of the five operational areas were three crews short of a full staff.
The NIAS said that while all emergency calls were answered, there were "many delays".
As a contingency, St John Ambulance staff were put on standby to cope with minor cases.
However, an ambulance spokesman stressed that this was a routine arrangement and St John personnel would never deal with emergency cases.
One paramedic said staff issues were the "only thing talked about" among personnel on the ground.
The medic of almost 20 years, who asked to remain anonymous, said: "They centralised how rotas are managed at headquarters, when it used to be managed locally by your manager.
"Now there is no flexibility, people are expected to work long shifts, like 10 weekends in a row, and are getting leave refused to cover sickness.
"Before, you used to be able to chop and change it about, now there is no movement and staff are taking sick leave when they really just need to swap a shift.
"People can't be sure they will even get time off they have booked long in advance and just don't risk it and call in sick."
He went on: "I've done 12-hour shifts and not eaten the entire time.
"It's rare to finish on time and then you are in early the next shift.
"We used to be very proud of our station and never had one shift dropped, there was always someone to cover, now there are shifts not filled all the time and it's hitting morale – it's shocking – I've never known it so bad.
"Calls are always answered, but I've known it to be the case that the nearest available ambulance is some 40 miles away."
The paramedic also said people were using the ambulance service to get treatment sooner than if they attended a doctor or hospital.
In a statement, the trust said: "The Northern Ireland Ambulance Service was disappointed that on Saturday, July 26 we had to operate at 15 crews short throughout Northern Ireland.
"That was three short in each operational area.
"During this time we responded to all emergency calls received, however, in many cases there were some delays."