Some people are over analysing the procedure of obtaining an Irish passport. Make no mistake, it is a valuable item and worth every penny it costs. Where there is a dispute regarding grandparent(s) etc I understand the trend is to come in favour of the applicant.
Both my grandfathers were from England. The "passport" situation doesn't work the other way round. I do not qualify for a UK passport. I applied for one some years ago and was refused within days of applying.
If you have a property in Spain and you have an Irish grandparent/parent/sibling and are a resident of the UK it probably is in your interest to get an Irish passport asap. There is ample information on the Irish Passport Office's website. Go for it and if you are in any doubt that you are in a small minority be aware that because of the huge amount of (UK) applicants it takes us Irish around six months even to renew our passports and even for that we have to jump through a number of hoops.
I keep saying this and still there are those who for whatever reason won't even think of applying.
Again, kudos to the administrators of this site for allowing such threads - some websites do not. And, if I may blow my own trumpet, I've been banging on this here for years.