The Garda warning was issued after thieves used the blockers to steal from cars in Co Westmeath. It's understood both of the victims had taken money out of the bank before they were targeted.
Gardaí in the midlands are now advising motorists to double check that their cars are locked after the two thefts in the Golden Island area of Athlone.
The signal blockers are used by thieves to prevent someone locking their car by remote control.
Signal jamming has been commonplace in countries like South Africa for some time and is now becoming a significant crime in the UK. Thieves watch as people park and, if drivers are using a remote key to lock their car as they walk away, the thieves interrupt the signal with the jammers. This means the car is left unlocked and the contents can then be rifled, including what's in the boot as this too is left open. The crime is particularly prevalent in car parks and shopping centre car parks, where several cars can be targeted at the same time. The jammers are widely available on the internet and can be bought easily and cheaply - less than €100 in some cases.
A South African insurance company, Safire, recently showed how the jamming technology works when one its clients were targeted.
Gardaí are advising motorists to check their car door handles after locking in order to ensure the car is secure. However, even manually checking the doors are locked can be an issue when they car has a keyless entry system - as many expensive cars now do. Once the key is about your person or in a bag the door will open automatically - adding to the challenge of establishing it is locked to begin with.
The National Police Chiefs' Council in Britain recently described the problem as a growing one. "It is a growing feature of vehicle crime. The Vehicle Crime Intelligence Unit is working closely with a number of partners, including the Home Office and motor manufacturers to prevent this crime now and in the future", Deputy Chief Constable, Matt Jukes, told the BBC.
The most important advice for drivers is to make ensure their car is locked and secure before they walk away from it and not rely solely on the remote key as they do so. Devices that alert drivers to the fact that their remotes are being jammed are also now available but following the manufacturer's or a dealers advice is possibly the best course of action.