Getting married
The main benefit for income tax is that you can share any unused tax credits or standard rate cut off with your spouse. This is only benefical if one spouse is earning over €36k and one spouse less than €36k. E.g. If you earned €20,000 and your spouse earned €50,000 the combined tax liability (before tax credits) would be €16,856. Under joint assessment the tax liability would be €14,966.
The other tax benefits is that any transfer of assets between spouses is exempt from all taxes.