Two men are being questioned over the discovery of bombing equipment in a van.
The vehicle was stopped by gardai investigating dissident republican activity in Monaghan town in the early hours of this morning.
Gardai said several items believed to be components for bomb making, including fertiliser, were found.
"These items and the van were seized for technical examination," said a Garda spokesman.
"Two males, 25 years and 50 years, were arrested and are currently being detained at Monaghan Garda station."
The pair, who are believed to be known to gardai, can be held for up to three days under section 30 Offences Against the State Act.