Europcar

Yes, many times, but not in Ireland.
It's one of the largest rental companies in Europe. No complaints.
I generally rent it through the following consolidators:
www.autoeurope.com
www.kemwel.com
(Now both part of the same corporate group)
The usually get you cars with the major companies cheaper than going directly.

They are very large rental company based in France with outlets in many European countries. I have rented from them in Portugal, Spain, France and Italy. I have found them to be professional with a selection of late model vehicles in very good condition. I used AutoEurope, a consolidator in the US, to arrange my rentals. Over all I have had only positive experiences with them and I recommend them. Just like American rental agencies, the Europcar counter personnel will try to have you buy additional insurance when receiving the car. Read your contract carefully because their terms are often more strict than those if you rented in the States. Return the car 2 minutes late and you'll be charged for an extra day.