V.similar issue has arisen for me Chewie.
Had cause to search for a secondhand car in Spain the last time I was there and, like Mommah says, these hold their value much more than they do here. The cars tend to last longer as well, I noticed Peugeot 309s, older Escorts and Renault 21s in daily use there that have long since disappeared from Irish roads. I only want a banger, but people were putting big prices on older cars there - particularly smaller cars.
My search hasn't yet resulted in finding a car, but I have learned a bit about the legalities around it and my understanding of it is that you must be Spanish (or at least resident there) to buy a Spanish-registered car in Spain.
If you are not resident or Spanish, then you can use your own car and it is legal to keep a foreign vehicle for certain periods of time in the country (assuming that you're Irish taxed, NCTd and appropriately insured to be there and can prove it).
I imagine that insurance here won't cover you to drive in Spain for very long and I have heard that Spanish insurance companies do stick the hand in for extra premiums on RHD vehicles, but it is possible to get insurance. I haven't tried this myself, so don't really know the ins and outs.
Using this method, the car has to come home every now and again to get its NCT so you'll be doing a long drive and an overnight ferry once every two years. You're taking big risks with the Spanish cops driving round without an NCT or Irish tax - they view it the same as the cops here and would be their excuse to take you off the road. Spanish authorities don't seem to like RHD at all.
That being said, I believe an RHD car can be imported into Spain by a foreign national who is resident once the car is tweaked with LHD headlights etc. It is subject to an import duty (said to be steep) and their national car test. I have heard reports that the Spain NCT testers may refuse to test an RHD car though, so the advice is to check with the local centre to see if they have an issue. One website I saw suggested it was only worth doing for expensive or unusual vehicles.
Do you have a Spanish national relative or know someone who's resident and in who's name you could buy a local car? Look for inspiration on Spanish Ebay and Kyero.es. In my hunt for a banger, I see a lot of first generation SEAT Ibiza's going for a song, but there might be a reason for that!
In the meantime, I've had several very good car hire deals out of Barcelona Airport using the UK price comparison site Travelsupermarket.