From age 65 onwards there is no charge to Income Tax provided your income does not exceed a certain threshold. For single and widowed persons this threshold is €18,000 for the year 2016 and for a married couple it is €36,000.
It is available for married couples once either spouse is aged 65 or over. If you have dependent children the threshold is increased at a fixed rate per child for each tax year.
You are tax free only if your income does not exceed the threshold. Where your income does exceed the threshold you will be liable for income tax subject to claiming what is called marginal relief which reduces your normal tax liability.