Like many before, it appears I have been 'had' by the above two companies. I booked through Car3000, two days rent and insurance and was told to pick up from Europcar. At Dublin, I was told Car3000's insurance was not valid so had to pay again.
Secondly, despite only using a fraction of the tank of petrol I was given, I only received a small % of what I was due back. Furthermore, I've just found out, that despite being told I needed change for the tolls, which I paid twice, I have just been billed for 2 auto toll fees that automatically are charged when using the car.
So, suggest anyone using the above companies, DONT. Plus, if you use anyone else, bear in mind all the scams that myself and other people on this site have been the victims when hiring a car at Dublin airport. Probably, cheaper in the long rung to get a TAXI !!