Quite wrong, I'm afraid. Firstly, it doesn't matter if he's from South or North Ireland, all are entitled to an Irish passport.
Secondly, an EU citizen does not get carte blanche to come to Spain and stay as long as they like. Whilst the 90 in 180 days does not apply, the maximum and EU citizen can stay in Spain in one trip is 90 days unless you sign on the register of foreign EU citizens resident in Spain. This is very e...
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...asy to do but must be done in the 3 months from arrival. This will allow you to stay in Spain for longer than the 90 days but, as you say, don't go over the 183 days or you become a fiscal (tax paying) resident.
You simply make an appointment with a foreigners' office or National Police station in the province you are going to stay in. You, with a UK passport, can accompany him with the same "spouse of" register but you must ensure you keep records of your travel as going over 90 days is only valid if you are accompanying him and you must be able to prove it. You will not be able to enter the Schengen area by yourself if you exceed 90 days in 180.
This link tells you all about it https://administracion.gob.es/pag_Home/en/Tu-espacio-europeo/derechos-obligaciones/ciudadanos/residencia/obtencion-residencia/inscribirte-residente.html
You make a prior appointment with the National Police at this link https://icp.administracionelectronica.gob.es/icpplus/index.html
It talks you through it by selecting National Police Procedures and then EU Citizen Registration Certificate. When completing the form ensure you tick De No residencia on the back. It also gives a link to the tax form code 790 Model 012 which you pay at a bank of about 12 euro (ish, it will tell you how much when you print the certificate).
You will require an in date passport, proof of funds (600 euro a month) and full health insurance coverage for your time in Spain which gives at least the same cover as the Spanish NHS with no copayments.
Northern Ireland and Southern Ireland are considered the same in the EU regardless of NI being in the UK