Detectives in the Republic believe dissident republicans from the border around south Armagh were behind Friday's raid on a branch of the Ulster Bank, in which more than €100,000 (£84,000) was snatched.
The heist took place at Dublin Road, Castleblayney, in Co Monaghan, as the money was being put into the bank's cash machine to meet pre-Christmas demand.
The raiders drove a cream-coloured van through a plate-glass window at the premises, after waiting outside for some time.
It is believed the two-man gang had handguns and they fled in the direction of Carrickmacross, before veering off towards Cullaville in south Armagh.
A security source said: "The raid had all the hallmarks of a military-style operation – there is little doubt that the dissidents were involved."
Gardai have appealed for information and have asked people to telephone Carrickmacross gardai or any Garda station.