You should try to work out what rate of tax will apply to your pension benefits at retirement. At present, a couple over 65 can earn up to €40,000 per year and be income tax exempt.
If your pensions would leave you under the income tax threshold, relief at 20% is an excellent deal as you're getting 20% tax relief and well as PRSI/Health Levy relief and paying no tax on the proceeds.
On the other hand, if your pensions or other investment income is going to be substantial, e.g. you have a portfolio of rented properties and/or dividend-paying shares, your income tax rate might be 41% in retirement. If this is you, getting 20% tax relief on contributions but being taxed at 41% on some of the proceeds doesn't add up. But this is quite rare as I rarely come across anyone who's likely to be on the high rate of tax in retirement but is at the low rate now.