Assuming we are talking UK, a fixed wing IR is not "valid" for a helicopter. Over the transition period to JAR FCL, the CAA are allowing those with existing IRs on either FW or RW licences to acquire IRs on the other sort of flying machine with exemption from the approved course. Since, for helicopters, this would be a 55 hour course, the exemption is definitely worth using. I believe that the CAA always insist on you having at least 5 hrs instrument time on the type to be flown on the test. They also still require a "pre-test" test to be carried out by an IRE before the flight test with a CAA examiner.