The service said it had a shortage of 24 staff - the equivalent of 12 ambulance crews - that could have led to delays for "less serious calls".
It said support was sought from voluntary and private ambulance crews.
The shortage was due to a factors including rostered leave, sickness, vacant shifts and cancelled overtime.
John McPoland from the NIAS said the shortage left the service about 20% down on the usual 60 crews it expected to have in place for a Saturday night shift.
He added that the staff shortage was an ongoing issue.
"We are experiencing difficulty at the weekend and we have been relying on the voluntary crews and on the private ambulances," he said.
"We also rely on our staff coming in and doing overtime.
"We have been dealing with situations like this for quite a while now.
"We're absolutely not happy about it but we will take steps to try to address it."
He added that the reduced service would mostly be felt by patients "whose situation is not immediately or potentially life-threatening".
The Northern Ireland Ambulance Service (NIAS) has said it had to operate a reduced level of cover on Saturday night due to a shortage of staff.