The IRS eliminates the lag errors associated with a traditional static-source-only based vertical speed indicator. Most transport category aircraft have "instantaneous VSI's" (IVSI). Using the IRS in conjunction with a static air source (through an Air Data Computer most commonly, I would suspect) makes the instrument more accurate. You might recall from your primary training days that it would take several seconds after initiating a maneuver for the VSI in a Cessna or Piper light trainer to begin showing a climb or descent indication. This should not be the case if an IVSI is installed.
The intricacies of the instruments' designs and their failure modes in various aircraft types exceed my knowledge level, however the manual for the aircraft I'm currently studying simply states:
Vertical Speed Indications:
Indicates ADC vertical speed as dampened by the IRS.