Republicans have been challenged to explain why explosives were not decommissioned after the discovery of a massive Semtex haul in the Republic of Ireland.
Garda said the explosives, found last week, were part of a cache donated to the Provisional IRA by Libya 30 years ago.
They were stored in an arms dump which should have been emptied and decommissioned as part of the peace process.
Ulster Unionist MLA Tom Elliott said republicans must explain why the explosives were not put beyond use.
He added: "We need republicans to tell us what exactly is going on here and why they have been holding on to explosives.
"It doesn't do much for unionist confidence, and we just wonder are they still part of Provisional IRA stock and are the Provisional IRA passing the weapons on to dissidents as and when they need them. We have long suspected that not all Semtex was decommissioned, and clearly there is still a lot of it about.
"I have always said that there is co-operation between dissidents and mainstream republicans and that is a major problem."
DUP MP Jeffrey Donaldson said the find caused him "grave concern".
He added: "It raises questions as to the credibility of the decommissioning process which was never put into the public domain.
"We said at the time we had no way of knowing if all weapons and explosives were handed over.
"We asked for proper verification including photographic evidence – that was denied.
"We were told the inventory of weapons decommissioned would eventually be published – that hasn't happened.
"When things like this happen, it does undermine confidence."
Fifteen kilos of Semtex and other bomb-making equipment, including detonator cord and igniters, were seized. The cache contained enough explosives to make more than 180 bombs.
In 2005 the senior IRA figure liaising with the Independent International Committee on Decommissioning claimed all its Semtex "was put beyond use".
At the time the British and Irish governments said a complete decommissioning had taken place.
The explosive was part of the arms dump found during searches at the Old Airport Road in Cloghran near Dublin last week.
Garda revealed more details of the cache on Thursday.
Firearms including an Uzi sub-machine-gun, a Glock semi-automatic pistol, revolvers, shotguns, a silencer and a Taser stun gun were also recovered, along with 1,300 rounds of ammunition.
A forensic analysis of the Semtex confirmed it had been concealed for over a decade.
According to one report, the weapons cache may have been stolen from the Provisional IRA during the Real IRA split in 1998.
Garda Superintendent Dave Taylor said there was no doubt the seizure had foiled attacks by dissidents.
He added: "These weapons are meant for one reason, and one reason only – to kill and maim and cause great destruction to the community.
"I have no doubt this will put a dent in the operation of these (dissident) groups."
Underlining the significance of the seizure, Garda said that just 80g of the explosive was used in the bomb that killed PSNI officer Ronan Kerr in 2011.
STORY SO FAR
The weapons and explosives haul was found last Wednesday during a search of lorry containers in a yard at Cloghran, north Dublin. It is unclear how long the cache was at the site but Garda believes the items were moved in recent years rather than stored at the same location since the late 1990s. Along with 15kg of Semtex, the weapons found included an UZI 9mm sub-machine-gun, revolvers, double-barrel shotguns, pistols, an air rifle and a Taser, plus around 1,300 rounds of ammunition.