I rented from Sixt in Dublin in September. First time I didn't have a reservation and went to their counter to ask about their rates for 2 days. They told me to go their website, find a rate, make a reservation and come back in 10 minutes so their systems would show the info. Unusual but worked fine. Reserved their smallest car and got upgraded to a nice Toyota Auris.

A week later, I had to wait for an hour at the airport. They told me the Compact car I reserved was a late return. I asked if there were any other cars available but nothing. Oh well, 1 hour wasn't too bad and they were significantly cheaper with included full insurance than any other company.

On a side note, US credit cards that usually provide rental car insurance coverage often exclude Ireland. And you'll be in for a shock if you have to add the insurance from the rental company at the counter.

No problems with the Sixt customer service, facilities or cars in Dublin. I would rent from them again.