Two brothers and an associate were detained on the N24 at Cahir in Co Tipperary on Thursday night, during a major police operation involving elite Garda officers.
Three hand-held rocket launchers - capable of blowing apart armoured personnel carriers and tanks - were taken away for further examinations. Two cars were also seized.
It is believed the swoop followed a surveillance operation targeting dissidents, and it is thought the weapons were en route to Northern Ireland.
A senior Garda officer said: "This is a significant capture.
"The intention was that they would go up north and would be passed on for further use."
The three men, who are aged in their 30s and are from Co Limerick, are being questioned at Clonmel and Cahir garda stations on suspicion of dissident republican activity, under section 30 of the Offences against the State Act 1939.
They are suspected of being involved in the Real IRA.
"We have full time surveillance operations going on all the time," added a Garda. "There came a point when we moved in and this one came to fruition."
Officers from the elite special detective unit, emergency response unit and other national and local Garda branches were involved in the operation in Tipperary.
The recovered explosives will be subjected to technical and ballistic examinations.
Local MLA Tom Elliott said the weapons could have been used to cause "death and destruction" had they crossed the border.
"Thanks to last night's Garda operation these dangerous weapons will not now find their way into the hands of terrorists and will not be used to cause death and destruction in Northern Ireland," the UUP justice spokesperson explained.
"These violent criminal gangs are a danger to society on both sides of the border and if the authorities in Northern Ireland are to successfully combat the threat posed by these criminal gangs, then it is essential that they continue to enjoy a close working relationship with the Garda and I would like to pay tribute to them for their vigilance and timely actions in disrupting this particular operation and seizing this cache of deadly weapons."