If you have an FAA IR or any other ICAO IR for that matter you will need to do a
minimum of 15 hours of training for the JAA IR according to a recent AIC (White) that was released in August 2002. See
www.ais.co.uk for details. After the 15 hours it is up to the FTO to determine whether you are ready to sit the test.
There are other schools in the USA which can train you for the JAA IR. You will not be allowed to do the flight test however outside of JAA member state airspace