They supplement the main scheme benefits, so they might or might not be depending on how much you have from the main scheme. Retirement lump sums totalling up to €200,000 can be taken tax-free from all pension schemes. Pension income is taxable but if it's small you might be below the tax thresholds.
If you're thinking about starting an AVC you need to speak to your Financial Broker to get them to show you the figures in your particular circumstances.