The fact that your bank is insisting on an NIE number the day you complete and that you are not going to the signing of deeds in person (and therefore can’t request your NIE at the same time) makes it all pretty complicated.
You are not legally required to have an NIE number to complete the sale (signing the deeds before a notary) as you can do this with your passport number. However if your mortgage lender insists on it then you have no option but to get one. It seems that some lenders insist on it and some don’t, though more and more are doing so.
Your problem is that you won’t have time to get an NIE number between now and 7th May, which is only 10 working days away. Even if you were to apply in person in Spain this Monday you almost certainly wouldn’t have it by the 7th May as it usually takes at least 15 days. If you were to apply via a consulate from abroad then it takes even longer.
By the way the 7th May is a Saturday and Notaries don’t work on Saturdays. Are you sure that this is the date for completion? What is actually going to happen on 7th May?
I suggest 2 possible solutions:
1) Delay the date of completion to give yourself time to sort out an NIE number. Is this possible?
2) Ask the bank whether they will accept proof that you have requested an NIE number and are waiting for it but not actually yet received it. This gives you time between now and the 7th May to request an NIE number, either in Spain or at a consulate in the UK. If they aren’t too bloody-minded they should accept this solution as you can prove that you have request your NIE number and will receive it in time to pay the necessary taxes (and inscribe the property and mortgage in the land registry, which is what really concerns them).
But if they accept this solution take care with requesting the NIE number from the UK as I don’t know how long it takes and it might not arrive in time, even if you were to request it straight away.
Regards your question as to whether there is a way you can apply from Northern Ireland the answer is ‘not really’. You will either have to go to London, Manchester or Edinburgh, or better still travel to Spain and do it yourself (much quicker result).
I say ‘not really’ because it is still possible in some parts of Spain to apply for an NIE number through a 3rd party, even without power of attorney. The law says you have to apply in person or have someone with power of attorney certified by a Notary do it for you. However these regulations are still ‘overlooked’ in some places. Get back to me on this if all other options fail you. But bear in mind that even if this were possible you would almost certainly not have your NIE number by 7th May, so it’s not really a solution if your bank insists on an NIE number for the 7th May.
You need to talk to you bank about your options. They want you to take their mortgage – it’s a competitive market after all – so I would imagine they would come to some sort of accommodation if you can convince them that you will have your NIE number in good time to pay the taxes (deadline 30 days after the signing of deeds).