I know a little about Jag IRSs. I put one under my T-bucket after doing quite a bit of research. If you google "jag irs" you'll get quite a few hits, many are on kit Cobra forums, as the Jag is very popular with the Cobra guys.
The XJ rearends are about 61" wide over the hubs. There should be a little metal tag held on by one of the differential cover bolts that has two numbers on it. The lower number is the number of teeth on the pinion gear. The higher number is the number of teeth on the ring gear. Divide the higher number by the lower number and you get your ratio, which is most likely 3.31, although there are some 3.54s out there. The differential is based on the Dana 44 ring, pinion and carrier, however the bearings are metric. The cover is standard Dana 44.
Setting up the IRS in your frame is not terribly difficult, but you need to pay close attention to the geometry and alignment. Assuming you aren't going to use the stock cage, you are going to need to fabricate a mount for the differential, keeping the pinion angle in mind. You will also need to fabricate upper shock mounts, front stabilizer struts and mounts, and radius rods and mounts. You'll also need front and rear tie-bars and a bottom plate to tie the lower control arm mounts together for strength. Your forward radius rod mounts should be in line with the inboard hinge axis of the lower control arms to prevent binding as the control arms move up and down in normal suspension travel.
Attached are some CAD drawings of my Jag set-up (done by the guy who fabbed my mounts).