NIE

I'm in a similar position. I have contacted the Spanish Consulate in Edinburgh , and have an appointment with them in September. They can't issue the NIE but will send off all of my completed paperwork to Madrid. At least that way I can still have the NIE until the citizenship issue is resolved. 

http://www.exteriores.gob.es/Consulados/EDIMBURGO/en/Consulado/Pages/LocalizacionContacto.aspx

An NIE is nothing whatsoever to do with nationality or when you enter the country.

If you're going to obtain Irish citizenship then you don't have to even think about Brexshite or Dec 31st.

Just a word of caution though. If you're hoping to acquire health care in Spain via an S1 then be aware that you only qualify to join the Irish health system after 12 months of residency and can't mix-n-match an Irish passport with UK S1. 

Thank you Ray, I'm not sure now what the withdrawal agreement is! If I arrived this year where would that leave me Jan 1st ?

If you arrive this year as a UK citizen, and don't initiate a residencia application in time, then that leaves you at the risk of having to go back to the UK and request a long-term visa from the Spanish consulate, for which you'd have to prove an income of about €25,000. 

Don't blame Spain or the EU - that's the sort of amount that Boris requires of EU citizens wanting to move to the UK.

The sensible thing to do would be to wait until you have an Irish passport.

Thank you Jim  I have been wanting to move for the last three years but there have been so many complications. I have to sell the house now I just don't want to be homeless! 

We bought a house in spain 15 years ago. We are now planning to semi retire there next year. We will be be living there 10 out of 12 months. We are not planning to apply for Spanish residency. Will we be staying there illegally?

Thanks for your reply. Does that mean that all expats now need to have Spanish residency? 

As the negotiations with EU have not been finalised yet surely that is still possible to change?