The Provisional IRA "pulled the wool" over British and Irish government eyes in its 2005 commitment that it had "put beyond use" all its store of the Semtex explosive, according to senior garda sources.
They now fear a large part or even the bulk of two tonnes of the explosive may have been spirited away by dissident republicans before the official decommissioning process took place in August 2005.
At the time, the senior IRA figure liaising with the Independent International Committee on Decommissioning (IICD) gave an assurance that all the IRA's Semtex "was put beyond use".
Since then, small amounts of the explosive – a gift along with large amounts of weapons from the Libyan dictator Muammar Gaddafi – have turned up.
But gardai now believe a substantial amount of the explosive may have been hidden away before the so-called decommissioning process began.
In 2005, the supposed decommissioning process was hailed by both governments who briefed journalists that complete decommissioning of weapons had taken place.
At the time of the supposed decommissioning, praise was heaped on the committee headed by Canadian general John de Chastelain.
Catholic priest Fr Alex Reid and former Methodist president Rev Harold Good observed what was said to have been the decommissioning of the IRA's weaponry – including the Semtex explosive.
In a statement afterwards, the two clergymen said: "We are satisfied that the arms decommissioning represents the totality of the IRA's arsenal."