An IRS for the S197 shouldn't be all that hard to do. You have plenty of solid mounting points under the rear of the car. It might cost some coin but there are plenty of IRS differentials to choose from and all you really need to do is build a rear subframe to house the diff and mount the control arms to. Again it would be spendy considering a tubular replacement for cars that come with IRS stock usually cost 2-4k without shocks differential or axles.
I highly doubt it would be effective to try to swap a used IRS from the S550. The amount of modification needed would probably make it nearly impossible to use that setup.
Stock 2016 EB
2013 GT Deep Impact Blue W/ Brembos
FRPP GT500 axle back, MGW race spec, Koni yellows, Eibach Pro Kit, GC camber plates, BMR LCA & relocation brackets, MRR-M350 19X10 Piano Black Wheels.
2014 Smart for Two.
1996 Mitsubishi Eclipse GSX. More mods than I can remember.