When I rented my Hertz car through easytourireland.com I asked them about the motorway tolls. I was informed that the only one you can't pay for at the barrier is the one on the M50 that I used going from Dublin Airport to the N4 exit. I was told that all the Hertz cars are registered with the toll authority and that the 3.10 euros would appear on my final rental invoice. They also told me that if I cross that toll plaza on my return on the last day that the 3.10 euros would be charged to my credit card on file about a week later.which it did! I paid the toll at the M6 before Galway and on the N18 before going through the tunnel under the Shannon River at Limerick. They had a coin basket where you can put in the exact change or sufficient coins to cover the toll and no change returned. There is also the option of going to the manned booth and getting change from a euro note. Hope that helps.