A bomb-making factory discovered by gardai near the border at the weekend is being linked to previous finds in farmyards in counties Kildare and Louth.
Anti-terrorist officers believe all three sites were being used by the self-styled Oglaigh na hEireann gang to manufacture mortars for attacks against security force targets in Northern Ireland.
More than 200kgs of fertiliser had been partly ground and bagged in preparation for mixing with other ingredients to manufacture homemade explosive.
The ammonium nitrate was discovered during a search of a farmhouse and outhouse at Hackballscross, Co Louth, by Garda Special Branch, backed up by other specialist units and local personnel.
Officers said it was likely that the haul was to have been moved to another location for the completion of mortar bombs for use, possibly during the marching season.
Detectives are satisfied there are close links between this seizure and another factory found in Co Louth last December.