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".

BBC Worldwide is expected to confirm the find at a press screening in London later this week.
It follows weeks of speculation that some lost episodes had been located.
A total of 106 episodes featuring the first two actors to play the Doctor, William Hartnell and Patrick Troughton, are currently missing.
The BBC destroyed many of the sci-fi drama's original transmission tapes in the 1960s and 1970s.
However, the majority of the episodes had been transferred on to film for foreign broadcasters. It is often these prints found in other countries that are the source of retrieved episodes.
In 1991, all four instalments of the Patrick Troughton adventure The Tomb of the Cybermen were discovered in Hong Kong.
The latest find comes as Doctor Who celebrates its 50th birthday. A special episode featuring the current Doctor Matt Smith and his predecessor David Tennant will be shown on the programme's anniversary on 23 November.
A raft of other programming to celebrate the long-running drama was also recently announced by the BBC.
Details of how fans will be able to watch the recovered episodes are also expected to be revealed later this week.

The pilgrims had been travelling through Mastung in Balochistan province on their way to the Iranian border when the attack happened.
Several other people were injured, some critically, Pakistani media reported.
No group has admitted carrying out the attack but police say they believe the motive was sectarian.
Sunni and Shia extremists have frequently clashed and launched attacks on each other over the past 20 years.
Bus driver Khushal Khan said his vehicle was stopped by eight to 10 men driving in two jeeps that came from the opposite direction.
Mr Khan said the men ordered everybody out of the bus and made them stand in a line before spraying them with AK-47 rifle bursts. Women and children were among the 45 passengers.
He said some managed to run away but others were shot. Six passengers who were injured were taken away for treatment.
Police said the bus had come from the provincial capital Quetta and was heading for the border town of Taftan.
It was the deadliest attack against Shias in Pakistan since a suicide bomber killed at least 57 people at a Shia rally in Quetta in September 2010.
Shia Muslims are a minority in Pakistan.
At least 13 people were killed when a suicide bomber struck in the car park of a Shia mosque in Quetta - capital of Balochistan province - during the Eid festival at the end of last month.
The BBC's M Ilyas Khan in Islamabad says that responsibility for most attacks has been claimed by the Lashkar-e-Jhangvi (LJ) sectarian outfit, a predominantly Punjabi group with links to al-Qaeda and the Taliban.
Police in Balochistan have over the years arrested a number of LJ activists in connection with these attacks. Some of them have been sentenced to death or imprisonment by the courts.
Our correspondent says both police and the Shia community believe the mastermind behind most of the recent attacks was an LJ commander, Osman Saifullah, who was arrested in 2006 but escaped from a maximum security anti-terrorism prison in the cantonment area of Quetta in 2008.
Buses leave Quetta every day to carry pilgrims to the Shia holy sites in western Iran and parts of southern Iraq.
They have been a target for many attacks by sectarian militants in recent years.
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.
[NEXT_CONCEPT]
A number of early episodes of Doctor Who, which were believed to have been permanently lost, have been returned to the BBC.
[NEXT_CONCEPT]
At least 26 Shia Muslim pilgrims have been killed by a group of gunmen who opened fire on a bus in western Pakistan, officials said.