The problem with this is Gardai aren't supposed to be like clamp happy private car park attendants. I see it as a bit demeaning for them and a further sign that the good relationship Irish Gardai have always had with the public is changing.
Stopping a car while the driver is available means that problems such as a tax disc in the post, delayed, accidentally stuck behind an old one, car turns out to be a clone etc. can all be discovered and dealt with.
While fine if someone has a disk more than a couple months out of date then sympathy would be low, my guess is that in practice out of tax cars will tend to be only a few weeks out. Unless someone never pays car tax there's no benefit to not paying as you always have to pay the tax since it was last taxed.
Using car parks means the driver gets no chance to justify themselves also the Garda will probably use the car registration database as the fine address, so anyone who's moved and not updated their address since buying their car isn't going to receive the fine.
Unless they attach a notice to the car on the spot - which I don't think they do - then the evildoer has no way of knowing a fine is due. The fines often get misdirected, the summons will eventually find it's way.
So it's much more likely to turn into a summons with all the expense and time wasting of a court appearance. Chances are quite high that any money brought in via fines will be lost many times over with the wasting of workers time in the courtroom.
Also there's an official period, 9 days or something where drivers can have an out of date disc. There's no guarantee a garda in a carpark to use this leniency as many will expect people who receive a fine out of the blue to just pay it regardless of valid excuses.
