I'd follow Clubman on this, and suggest a quick e-mail query to the Revenue here, on the double taxation issue. You'll probably take a bigger 'hit' on the Spanish side than you would normally expect to at this end, but I'm not familiar enough with the relevant legislation to be sure about that. You might well be entitled subsequently to some sort of tax refund here, depending on residency details...(?)
As for the cheapest way to actually transfer the dosh - I do know that, as of July 2004, banks operating within the Eurozone must provide euro-denominated transfers to receiving institutions (again within the Eurozone) at no greater cost than they would charge for domestic transfers - as long as the sum is <€12,500*, you can supply full IBAN & BIC/Swift details for the receiving institution, and you're not looking for some sort of express, 'same-day' transfer arrangement. So I'd suggest you carve it up into 5 separate transfers for €12,000 each. But have the relevant party in Spain enquire as to precisely what their own charges will be for this. Spanish banks are notorious for being a bit of a law-unto-themselves on this kind of thing, and you (or someone) may have to bang the desk a bit and insist on your EU rights..!
I'll try and post links to the relevant legislation later. In the meantime, don't be bullied into forking over exorbitant bank fees for something that shouldn't cost more than (5x) €0.50~ €1.00 a pop.
(* from July 2006, this ceiling will be raised - coincidentally! - to <€60,000...)