Counting the runway lights is always the more accurate method of determining Visibility along the runway.
IRVR is a nice piece of kit, but it only takes a very localised reading at each site, and the algorithm converting the light reading into RVR always assumes a level of contamination (dirt, oil etc) on the AGL light fittings. Hence if your airport has a decent AGL cleaning regime, then your IRVR is probably probsbly reporting lower RVRs than is actually the case.