Will need to apply for residency after the initial 3 months and concerned if it will be a problem to have UK passport no on rental contract, rather than the Irish passport no. I will be using the Irish passport to travel so that I can work as a EU citizen. Need to sign rental contract asap but Irish passport hasn’t yet arrived - been printed and dispatched
Will need to apply for residency after the initial 3 months
I am not familiar with the details of the Spanish process, but in general you should apply before the the end of the three months, not after.
and concerned if it will be a problem to have UK passport no on rental contract, rather than the Irish passport no.
It should not be a problem. If anyone ever questions whether your Irish passport and your rental contract both belong to the same person, you can always show the UK passport, which is mentioned on the contract and has the same biographical details as the Irish passport.
I will be using the Irish passport to travel so that I can work as a EU citizen.
You don't need to use the Irish passport to travel. Your right to work comes from your being an Irish citizen. Even if you travel with a UK passport, you can use the Irish passport to show that you have a right to work, whether to your employer or to the Spanish government. (In theory you can even prove your Irish citizenship using any means whatsoever, but in practice the passport or a passport card is the only convenient way to do it.)
If you have an Irish Passport Card, this can be used instead. It is treated the same as other EU countries' ID cards. It costs about half the price of an Irish Passport
It lasts 5 years, or to the expiry of your Irish Passport, whichever is sooner. If you renew your Passport Card with only a few months remaining, then you can keep the old card - you don't have to surrender it to the Irish Passport Authority.
That way, in 10 years time when both expire, you can keep your (about to expire) Passport Card while you renew both your Passport and Passport Card.