IRS stands for Independent Rear Suspension, as opposed to Swing Axle which was fitted to pre-1967 Buses, and early beetles.
Its common to convert Splitscreen Buses to IRS rear suspension, this has several advantages.
- avoids camber change so allows suspension height to be lowered/adjusted neatly
- gets rid of the reduction hubs so improves gear ratios
- improves handling, as the Swing Axle tendency for an inner rear wheel to 'tuck under' when cornering heavily is removed.
There are several methods of converting a bus to IRS, either by using bespoke new parts (Creative Engineering produce a complete kit of parts for this), or by adapting/modifying parts from bay window buses.