John:
I am stating what the situation was before Brussels IV where EU regulations had no word on cross-border wills/inheritances. It is quite sensible, things will turn out to be as they were when the matter was just regulated by General International Private Law unless Spain-UK agrees specifically on this.
As an Ireland national, yes, you can grant a will in Spain where even being Spain resident, you can choose your nationality to govern your post-mortem affairs, if not, by force of Brussels IV, Spain inheritance Law will apply.
If the will was made according to Brussels IV, of course, its regulation will remain, even after Brexit as those will be acquired rights. I am saying this by virtue of General Principles of Law, just if you wonder how I can be certain.
So, for those Spain residents, owning real estate in Spain and no real estate in the UK, who will want UK to apply to his inheritance, it is highly advisabe to grant a Brussels IV will now, before the Brexit process is concluded