Five men have been arrested in Co Louth on suspicion of trying to move explosives over the border.
It is understood shots were fired at officers from the Special Detective Unit and the Emergency Response Unit, but there were no injuries.
The incident happened when gardaí stopped three cars in the Omeath area as part of an investigation into dissident republicans.
The five men are being held in garda stations in Drogheda, Dundalk and Balbriggan.
They can be questioned for up to three days.
The three cars have been seized and are undergoing a technical examination.
Gardaí have found a quantity of ammunition in counties Louth and Meath in follow-up searches.
Ahern takes dissident threat seriously
Minister for Justice Dermot Ahern has said he takes the threat posed by dissidents very seriously.
The Louth TD also congratulated gardaí on the success of their operation.
He said he had been briefed this morning by Garda Commissioner Fachtna Murphy on the ongoing operation.
The Minister said: 'The gardaí and the PSNI are working very closely together as is evidenced by what unfolded in the last 24 hours.'
Mr Ahern met Northern Ireland Justice Minister David Ford in Carlingford yesterday.
They reviewed policing co-operation and the ongoing crackdown on cross-border criminality.
