In short i am not sure but Halfords in Liffey Valley have a good selection of bike racks and ones you can put on the roof (if you have roof rack) that are very handy. Might be worth giving them a ring.
They have one in Argos (Dundrum Shopping Centre). In the catalogue it looks like it is ony for hatchbacks. I bought one on that basis but when I took it home it was only for saloon type cars. So check that out before you leave the shop because even the picture in the catalogue was incorrect. I use a hatchback type alot and as far as I know it does not obscure lights or number plate
ophelia, just to clarify, the rack in argos is for saloon type cars only? I knew that hatchback models didn't obscure the lights etc but that's because of the design of the car allowing the bikes to be higher and at an angle.