If you mean _just_ income tax, then, given that low-band income tax is 20%, it should be the tax credits the person is entitled to times five.
So for instance if they're single and PAYE in 2011, they have a tax credit of 1650 + 1650 = 3300 euro, and they'll pay no income tax on the first 16500 euro. However, they'll still pay some PRSI and USC. There shouldn't be any particular benefit to staying under this number; if you earn 16501, then you pay 16501 * 0.2 = 3300.20 - 3300 = 20 cent income tax.