Re: Irish passport
When (if) Brexit happens and assuming you can't get Spanish nationality because of ancestry, you could get residency as a non-EU family member. Then you would need 10* years of constant uninterrupted residency to get Spanish nationality. You would need to be able to speak Spanish and have paid social security stamps for most of that time.
You would need to give up your British nationality but Britain allows you to renounce your British nationality to gain Spanish (or any other) nationality and then immediately apply to recover your British nationality again.
* Yep, 10, unless you have Spanish grandparents or ancestry from South America, Andorra, Philippines, Equatorial Guinea, or Portugal, or Sephardic Jewish ancestry in which case it's two.