Alex,
I don't think the IVSI will work with only IRS input! Air data is definetely required!
If you only used IRS, the VSI would drift over time (just as GS/position do!)
Using the IRS only for tendency/acceleration info (basicly the I in IVSI)
Without Air data the IRS would fluctuate around 0 as the aircraft climbs/descends. But it won't show a correct value.
Regards,
Mark