We are renting a car from Europcar from AutoEurope which we chose Toyota Corolla or similar, basically a "compact" car. Does anyone know what actual car model you get for this? What are the chances it will use diesel?

I think it depends entirely on what cars are available. Last year I rented a car in Ireland through AutoEurope and chose the same category you did. My car was provided by Budget and it was a gasoline-powered Ford Focus hatchback. In France the car came from Europcar and was a diesel-powered Mercedes hatchback (a pleasant surprise!).

Unless you pay a premium price for a specific model car (like a sports car or super luxury) you get whatever they happen to have available in your class or greater. Have no idea which fleets have what % of diesels.

And in Europe Corolla is not compact - it' considered Intermediate by rental companies in the US and that or full-size sedan in europe.

I was in that category, and got a Fiat Punto. It was unleaded, not diesel. It fit 3 fat ladies, and their luggage fine, which was three large checked pieces and three carryons, plus purses.

I just returned from Ireland on Monday. We rented a car from Europcar from Auto Europe. It was a Toyota Corolla hatch back. It worked out great. I wouldn't want anything bigger. The roads are narrow. We were even ran into the ditch by another car when they were passing us on a very narrow road. Thank goodness my sister threw it into reverse and we got out, but it was in the middle of nowhere and we could have been really stuck.