Dr. Declan Bedford:

People can buy breathalyser-type machines themselves, however, the difficulty is whether we can depend on them. Are they calibrated? The machines that the Medical Bureau of Road Safety give out to the Garda are checked at least twice a year and tests are done to make sure that they are correct. The difficulty here is whether we could depend on breathalysers that customers might use in the pub themselves. For starters, if somebody had just finished a drink the machine would be likely to give a higher result straight away. It might also not be calibrated correctly and might give a false negative. Somebody getting a false negative and then heading out in the car would produce a very bad situation. A positive might be fine and might lead someone to decide not to drink and drive. There is a risk, however.

I do not know of any studies done at a population level to state whether it works.