Quite simple really. As the law currently stands, when I am driving, and I am stopped at a formal Garda checkpoint, there is rarely any need for the Garda to (1) ask me to get out of my car; or (2) detain me for a period longer than a minute or two. A 10-second conversation will give a Garda a very good indication as to whether or not I have drink taken.
Random breath testing exercises will (presumably) require drivers to (1) wait to be tested if there are other drivers being tested in front of them; (2) get out of their car to be tested; (3) wait while the Garda reads them the standard statutory warnings etc in advance of the test; (4) undergo the test itself; (5) wait for the results, and presumably some sort of written confirmation that they have passed (which I presume will be necessary to protect both the interests and rights of the driver and of the Gardai involved in the testing procedure).
All of this will take some time and it is reasonable to expect that the process in each individual case could take maybe 5 minutes, maybe longer. This would mean in practice that each individual Garda could only test 10 or 15 drivers an hour. Even at the most optimistic estimate, a Garda will not be able to test much more than 30 drivers an hour, or one every two minutes.
Now consider that most stretches of national road carry over 20,000 vehicles per day, some a large multiple of this figure. This equates to roughly 1,000 vehicles per hour on average. To test 30 drivers out of an average of 1,000 per hour means that 97% of drivers would be flagged through without being checked at all (which defeats the purpose of the exercise in the first instance as a drunk has only a 3% chance of being caught. Even at a checkpoint.) Otherwise, large traffic jams would ensue as drivers would be forced to wait interminably to be tested.
Even at night when road use would be only 10% of the daily rate, a drunk driver would still stand a 70% chance of being waved through.
I honestly can't see any merit in this proposal.