As far as I recall, when doing the FAA IR you can bring in up to 25 hours of previous instrument instruction, provided each flight is signed off by a CFI. Sadly I don't recall where I saw that requirement but it's very useful; a typical IMC Rated pilot will have done 20-30 hours of training (no, 15hrs is not enough) and that takes care of most of the mandatory IR flying hours, and provided you are current etc getting the FAA IR should not be too hard. FAA IRs I know have confirmed this.
But what credit does one get towards the JAA IR?