Your AVC will come off your taxable income, and where you are paying at top rate then this means you have less income taxable at top rate (i.e. 42% relief). I presume you are doing a tax return to claim the relief - a form for PRSI refunds recently went up on revenue.ie - check the Whats New section.
To play with the numbers you could pop in to any Revenue office and get a ROS CD (its free), then download the Form 11 for 2005 and away you go calculating your income tax - you can mess around with this to prove to yourself that you should be getting top rate relief.
To actually file this way you need to go through registration steps which take a few days - also by filing a Form 11 you are making yourself out to be a "chargeable person" - i.e. someone who is obliged to make a return. There should (you'd expect anyway!) be a Form 12 (for PAYE workers) up on ROS by next October. However, theres nothing to stop you just using the Form 11 as an accurate tax calculator - you dont HAVE to register etc. etc. just because you get the CD and download Form 11.