You should apply to your local Inspector of Taxes for a Certificate of Tax Credits and Standard Rate cut off point, quoting your ARF provider as your "Employer". You should quote their tax reference number which they should be able to provide.
The Revenue can then determine what, if any, tax credits can be allocated to this "employment" and will put the details on ROS, which the ARF provider can then access and will be able to calculate how much tax to deduct. As Conan points out, there is also a 2% Health Levy, but only if you are under age 70. If you are 70 or over you do not have to pay the Health Levy.
All the above assumes your ARF commenced on or after 06/04/2000. If your ARF commenced before this date, you will have to declare this income yourself and pay tax yourself after a self assessment