Firstly, welcome to the forums from a fellow european. Can't really help you on the import questions, seeing I never imported a car myself

. The original location of the car isn't that important either, it's the guidelines you need to adhere to in Spain.
Your best bet would be to inform yourself of the technical requirements that the spanish commision of road vehicles puts on an oldtimer. Then verify what the car has, and what needs altering. That'd give you a pricetag on the actual technical changes needed to get the car approved for Spain. They will also be able to inform you of the cost of putting a license-plate to the car (the administration of that stuff can also be somewhat hefty in comparison).
Also contact customs to verify if any costs are present when actually 'importing' the car, such as taxes.
Also make sure your insurance will cover the car (nothing more of a hassle, than having the car, and not being able to drive it cause it's not insured).
Other then that, I'm guessing you'd be able to drive the car over to Spain from Ireland, so shipping shouldn't really need to factor into the equation. Do make sure tho the car is ready for a longer trip if you do it this way: tire-pressure, coolant, temperature, all that kinda stuff.
Ofcourse there maybe others on the forums that will be able to further inform you about things, but this little list should get you through the basics. And ofcourse contacting the various instances may provide you with information on who to call, and what to do to make it all go smooth.
