Irish citizens getting married in Spain should check with the local Registrar where they intend to get married with regard to their documentary requirements. In particular, you should check whether the Registrar requires Apostille stamps on the Certificate of Non-Impediment and Irish birth certificate. If you are an Irish citizen who was born outside of Ireland, we cannot place an Apostille on your birth certificate - this must be done by the government that issued your birth certificate.
If the Registrar requests a Certificate of Residency, stating that you have lived in the UK for a certain length of time, you should contact your local council in Britain for electoral records. The Irish Embassy in London does not hold records on the residency of Irish citizens who are resident in the UK.