I'm not aware of a pre-pay facility but I'm sure if you're paying at the shop and travelling through the toll a few minutes later it shouldn't matter. Note that you need your car's registration plate details so get those from the car hire company before paying.

The toll can be paid at any shop carrying the "Payzone" facility (look for the bright pink signage). According to www.payzone.ie the Esso petrol station on the way out of the airport is set up so this may be the handiest for you