If you have IRS, the only 'spring' is that silly transverse composite material leaf spring. The IRS would need additional support as the Nivomat dampers include a spring in the assembly.

I cannot understand why you would use IRS with such soft front and rear spring (sprung) suspension. I'm sort of critical of the IRS notion, to deliver a sort of adjustable rear wheel camber so the tire tread width maintains optimal contact with the road surface on, well, more spirited maneuvers. The rear axle half shaft on the IRS use constant-velocity joint sealed inside flex boots and these would benefit from inspection, cleaning, and re-lubrication on occasion. The solid live axle on later 700 and 900 series have rear suspension add ons, such as the cage secure to the front of the differential, with extra bushings, that require inspection and replacement. Critical of the design, I am.

Anyhoo, some diagrams for you to help you:

Here is on diagram of the left side 97-98 IRS when viewed from the rear:



From here:
https://volvoforums.com/forum/volvo-s90-v90-40/rear-suspension-bushing-question-74426/

The Volvo parts webstore page seems to corroborate your assertion that 97-98 S90s so equipped with IRS may or may not have self-leveling as that is an option.

As clear as mud for me.

http://www.swedishautoparts.com/S90/Volvo-S90-suspension-rear.html

So, the the non-Nivomat 97-98 rear IRS would need rear dampers with spring assist, like coilovers? Or would these use coil springs in addition to the leaf spring.

I'm unsure.
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