If you're treated singly you will pay 20% on €36,400 and your spouse on €30,000 (as she only earns €30,000)
Jointly assessed-one person can earn up to a maximum of €45,400 at 20% and the spouse can earn up to €27,400 at 20%, therefore collectively you can earn €72,800 at 20% instead of €66,400.
Check out the 'just married' leaflet on ww.revenue.ie