If both of you have taxable income, you can decide which of you is to be the assessable spouse. You then ask the tax office to allocate the tax credits and standard rate cut-off point between you in whatever way you wish. (The PAYE tax credit, employment expenses and the increase in standard rate cut-off point of 23,000 euro are not transferable).
If your tax office does not receive a request from you to allocate your tax credits in any particular way; the tax office will normally give all the tax credits (other than the other partner's PAYE and expense tax credits) to the spouse being assessed. The spouse being assessed must complete the return of income for the couple and is chargeable to tax on the joint income of the couple.