If they are made via payroll then tax and PRSI relief will be granted at source (i.e. your tax and PRSI will be calculated on your gross less any pension contributions including AVCs).
If you make them off payroll then you need to write to Revenue and claim the tax and PRSI back yourself. I think they will process this as soon as you claim (allowing for time for them to process the claim) rather than at year end or Oct 31st.
Also don't forget to claim for the PRSI element (up to 6% if your income in below the PRSI limit). You have to write to the PRSI section of the Collector General in Limerick although lately I've heard that the details of the claim are automatically passed to this section.