I bought a car last June, due to some mistakes by the selling garage it didn't get transferred into my name until November. I sent a cheque to dublin city council for the period June 06-November 07, they wrote back and said that as the car was only transferred into my name in November that was the date i had to pay from.....
during that 5 months i was stopped at two checkpoints and it wasn't noticed, about a week before i finally got my disc i had to explain to a garda who was taking my details (disc out of date since April 07) outside the office what had happened, he accepted my story.
As soon as you get proof or regsitration back from Shannon you can send the form to the local authority. That is technically how long you can "get away with it".