In answer to a question on our facebook group and from recent personal experience and assuming you are considering switching your nationality on official documents from British to Irish.
To change nacionalidad I filled EX 18 and in the modification area wrote Cambiar Nacionalidad. However this is not widely recognised and in your position you may wish to use in modification – change of passport and then add your Irish passport details. I completed the EX 18 as an Irish citizen with my Irish passport details and included same in appendix. I made s cita for the Oficina de Extranjeros in normal manner. On first visit my application was unsuccessful but not dismissed and I was asked for a certificate to prove Irish citizen entitlement. The Irish Embassy do not provide such a certificate but did give me a letter explaining the situation. However I also received a call from Oficina de Extranjeros to come back in as they had researched my application and it was granted. I was given appointment and had to produce padrón, Irish passport, proof of healthcare and pay the tasa. I was then given a new Tarjeta. You may be asked for a Certificado de Concordancia and will attach link re this. This was in Granada. After receiving my new residencia I informed my padrón office and bank producing both residency certificate and passport.
LINK TO ‘HOW TO MAKE APPOINTMENT’ POLICIA NACIONAL – EXTRANJERÍA