That'a a little frustrating alright.

There are two Hertz facilities close to the city centre - if you're close to that Avis facility I suspect you'll be close to the Hertz facility on South Circular Road. You maybe wedded to using Avis but I've often done a one way with Hertz to the airport and I've never been charged the one way fee.

By the way, for most car classes, renting in Ireland is much more expensive than the US - sigh...